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Chemistry 227 228 and 229 General Chemistry Labs Lab Coordinators Dr Eric Sheagley shea2leypdxedu Dr Dean Atkinson atkinsdb pdXedu Table of Contents General Chemistry Laboratory 3 Grading Criteria 4 Laboratory Safety Rules and Procedures 5 Keeping a Lab Notebook 9 Report Guidelines 14 Example Lab Report 18 General Chemistry Lab Report Checklist 21 CH227 LABS 23 PreLab Who has the same solid that I have 23 PreLab How much sugar is in a can of coke 30 PreLab A Cycle of Copper Reactions 37 Prelab Which Alkali Metal Carbonate 43 Prelab Using Conductivity to Find an Equivalence Point 48 Prelab Atomic Emission Spectra 53 PreLab Determining the Concentration of a Solution Beer s Law 62 Electron Density Prelab 68 CH228 LABS 70 PreLab Enthalpy of Neutralization of Phosphoric Acid 70 PreLab Hess s Law 75 Deriving the Gas Laws Using Computer Simulations 79 PreLab Decomposition of Hydrogen Peroxide 85 PreLab Vapor Pressure and Heat of Vaporization 90 PreLab Using Freezing Point Depression to Find Molecular Weight 96 PreLab The Rate and Order of a Chemical Reaction 102 PreLab Chemical Equilibrium Finding a Constant Kc 107 Le Chatelier s Principle in a Cobalt Complex 114 CH229 LABS 117 Acid Rain 117 Acid Dissociation Constant Ka 122 Titration of a Diprotic Acid Identifying an Unknown 126 Buffers 134 Determination of the Ksp of Calcium Hydroxide 140 Thermodynamics of the Solubility of Potassium Nitrate 146 Redox Titration Analysis of a Commercial Bleach 151 Pre Lab Electrochemistry Voltaic Cells 156 Synthesis of Acetaminophen 162 General Chemistry Laboratory SYLLABUS Winter 2012 Lab Packet All printed material for this lab will be available on D2L under the heading labeled Course Content OR may be purchased at Smart Copy 1915 SW 6th Avenue Prelab Exercises Prelab instructions are included in the lab packet You should answer any questions presented and prepare for the weeks lab before your lab meeting Pre labs are due at the beginning of the lab period Materials You will need chemical splash safety goggles These are available from the chemistry stockroom Room 280 SRTC or at the campus bookstore You will need a bound carbonless copv notebook not loose paper for recording data You are responsible for all laboratory equipment checked out to you If you break glassware you will pay the replacement cost of the glassware Dress for lab You must wear shoes that cover your entire foot including the heel They should fit up near your ankle leather is preferred but any nonporous material is okay Short shorts and short skirts are not allowed Your clothing must cover your torso and legs down to your knees You will also be required to wear a provided lab coat while working in the lab Grading The laboratory is graded on a Pass No Pass basis An average of 75 of all points available in the lab is required to pass Late Work Laboratory reports are due at the beginning of the lab period following completion of the experiment Lab reports should be typed Late reports will be docked 5 points per day late Attendance Attendance in this lab is mandatory YOU MUST ATTEND ALL SCHEDULED LABORATORY MEETINGS If you are not able to attend lab you must notify your laboratory instructor as soon as possible Students are responsible for completing the lab report for the missed lab Data can be obtained from a lab partner or the lab TA The made up work should be clearly labeled and indicate the origin of the data reported Reports are due the class meeting following the syllabus deadline In addition to completing the makeup lab you mist make up the missed lab time The makeup laboratory will not be the same lab you missed but will be a unique activity that will take place during week 10 of the quarter during the regularly scheduled lab period FAILURE TO DO BOTH WILL RESULT IN A NO PASS GRADE If you miss two or more labs your grade will be a NO PASS NOTE If you are more than 15 minutes late to lab you will be marked late Two late arrivals during the term will be counted as a missed lab In addition late students may be assigned to lab clean up duties at the conclusion of the lab period If you are chronically late you will be given a NO PASS at the lab coordinators discretion Plagiarism Experiments will be done in groups sharing the computer for data analysis and acquisition You may compare data with other groups but the content of your lab reports MUST be written individually It will be considered an act of plagiarism if you borrow tables or graphs from another student learning how to properly create a table or graph is an important skill learn how to do it on your own You cannot paraphrase the internet your book or any other source without the proper reference Additionally it will be considered an act of plagiarism if you borrow data without prior approval from your TA There are additional resources online to help you avoid plagiarism Please be sure to check httpWWWlibpdXeduinstructionsurvivalguideWriteandcitemainhtm or httpWebpdXedub5mgplagWebhtml and feel free to discuss the issue with your TA or the lab coordinator Depending on the severity of the offenses you will receive at a minimum a zero score for the report Additionally a report may be made to the Office of Student Affairs Grading Criteria Unless otherwise noted every lab report is worth 90 points including the prelab notebook and technique Each lab report will be graded according to the following point distribution Prelabz 10 points Abstract 10 points Introduction 10 points Data 10 points Results 15 points Discussion 15 points In addition to the above points each lab meeting will have an additional 20 points assigned on the following basis Notebook 10 points These points are awarded by the TA based upon the quality of your lab notebook Your TA will be looking to see that you are including a title a statement of purpose the procedures data tables and that all data is present Lab technique 10 points The basis for assigning these points includes but is not limited to general lab technique and methods safety general mannerism in lab and cleanliness Both of these criteria will be evaluated by your TA during each lab meeting At the end of each lab you must check out with your TA so that he or she can assess your lab notebook and verify that you have cleaned your work area Labs are graded on a PassNo Pass basis In order to pass the lab you must turn in every lab report More than one absence will result in a No Pass for the class You must receive 75 or greater of all the points available to pass Do not copy your partners friends old lab reports That is plagiarism Laboratory Safety Rules and Procedures Safety Rules The guidelines below are established for your and your classmates personal safety Failure to adhere to the guidelines below will result in a loss of Lab Technique points Personal Protective Equipment PPE is used to protect you from serious injuries or illnesses resulting from contact with chemical hazards in the laboratory Spills and other accidents can occur when least expected For this reason it is necessary to wear proper PPE The PPE for student labs consist of goggles gloves and clothing Proper PPE is required for all students or they will be asked to leave the lab Goggles Goggles must be worn whenever any experimental work is being done in the laboratory to protect the eyes against splashes Only indirectvented goggles are allowed in the student labs and should be wom at all times when any chemical is being used in the lab These are for sale in the bookstore and stockroom You should not wear Contact lenses in a chemical laboratory Chemical vapors may become trapped behind the lenses and cause eye damage Some chemicals may dissolve soft contact lenses The most important aspect of having the goggles fit comfortably is the proper adjustment of the strap length Adjust the strap length so that the goggles fit comfortably securely and are not too tight If you find that your goggles tend to fog you can pickup antifog tissue from the stockroom Gloves Gloves should be worn to protect the hands from chemicals Gloves are provided through your student fees and are located in the student labs For health and safety reasons it is important to always remove at least one glove when leaving the student laboratory this prevents things such as door handles from getting contaminated Clothing Dress appropriately for laboratory work You must wear shoes that cover your entire foot including the heel They should fit up near your ankle leather is preferred but any non porous material is okay Your clothing must cover your torso and legs down to your knees In addition you are required to wear a department provided lab coat while working in the lab Eating drinking and smoking are prohibited in the laboratory at ALL times Wash your hands after finishing lab work and refrain from quick trips to the hall to drink or eat during lab If you take a break be certain to remove gloves and wash hands before ingesting food or drink Never work alone in the laboratory or in the absence of the instructor Headphones may not be worn in lab Safety Procedures Know location of safety equipment re extinguisher re blanket rst aid kit safety shower eyewash fountain and all exits In case of fire or accident call the instructor at once Small res may be extinguished by wet towels If a person s clothing catches re roll the person in the fire blanket to extinguish the ames In case of a chemical spill on the body or clothing stand under the safety shower and ood the affected area with water Remove clothing to minimize contamination with the chemical If evacuation of the lab is necessary leave through any door that is safe or not obstructed doors that lead to other labs may be the best choice Leave the building by the nearest exit and meet your TA on the field next to Hoffmann Hall This would also be the meeting place in the event of an earthquake or other emergency It is good to know the nearest exits of your lab on the first day of class Spilled chemicals must be cleaned up immediately If the material is corrosive or ammable ask the instructor for assistance If acids or bases are spilled on the oor or bench neutralize with sodium bicarbonate then dilute with water Most other chemicals can be sponged off with water Avoid contact with blood or bodily uids Notify the instructor or stockroom personnel if ANY blood is spilled in the lab so that proper clean up and disposal procedures may be followed If a mercury thermometer is broken do not attempt to clean up yourself Notify students around you so that mercury is not spread then notify your lab instructor or stockroom personnel The stockroom is equipped for proper clean up and disposal of mercury Laboratory Procedures and Protocol General Etiquette Leave all equipment and work areas as you would wish to nd them Keep your lab bench area neat and free of spilled chemicals Your book bag coat etc should be kept in the designated area at the entrance to the lab not at your bench All chemical waste must be disposed of in proper containers Proper disposal of chemicals is important for student safety and proper disposal Putting chemicals into the wrong containers can lead to injury from unexpected chemical reactions Mixing waste can also make it more difficult or expensive for PSU to dispose of them Only chemicals should go into waste jars Waste jars for each experiment will be provided in the lab They will be labeled specifying which contents should be placed inside It is important that you replace the lids to the waste containers When done with the waste jar make sure it is placed in a secondary container Do not put anything down the sink unless you are explicitly told to dispose of it this way Your instructor will provide specific disposal guidelines when needed Following these guidelines assists us in lowering the environmental impact of the labs There are several locations for very specific waste i Chemical Waste these containers are ONLY for chemical waste generated in the lab They are each specifically labeled for each lab and waste type READ THE LABELS ii Contaminated paper Waste this is ONLY for paper towels used for cleanup of chemical spills iii Broken glass this is ONLY for broken glassware iv Gloves this is ONLY for used gloves v Normal trash this is for all other trash that is not chemically contaminated glass or gloves Clean your bench and equipment Clean all your glassware dirty glassware is harder to clean later Wash with water and detergent scrubbing with a brush as necessary Rinse well with water Do not dry glassware with compressed air as it is frequently oily The water and gas should be turned off and your equipment drawer locked Clean the common areas before you leave the lab Point deductions for the entire class will be imposed if the instructor or stockroom is not satisfied Return any special equipment to its proper location or the stockroom Handling Chemicals Obtaining reagents 0 Read the label CAREFULLY The Chemicals are organized by experiment in secondary containment bins Make sure the chemical name and concentration match what is required by the experiment Do not take the reagents to your bench We recommend always picking up bottles by the label If all students do this then any unnoticed spills when pouring will not cause possible problems for the next user Remember to wear gloves while working with reagents Do not put stoppers or lids from reagents down on the lab bench They may become contaminated Be sure that the lids or stoppers are replaced Do not place your own pipet dropper or spatulas into the reagent jar Pour a small amount into a beaker and measure from that Please pour on the conservative side to minimize waste and cost of labs You can always go back for more Do not put any excess reagent back in the reagent jar Treat it as waste and dispose of it properly When weighing chemicals on the balances never weigh directly onto the weighing pan Weigh into a weighing boat or beaker Any spills on the balances MUST be cleaned up immediately If you are unclear how to clean a spill notify your instructor The balances you are using are precision pieces of equipment and costs up to 4000 All chemicals should be treated as potentially hazardous and toxic Never taste a chemical or solution When smelling a chemical gently fan the vapors toward your nose Any chemicals that come in contact with your skin should be immediately washed with soap and copious amounts of water Laboratory Procedures Never pipet any liquid directly by mouth Use a rubber bulb to draw liquid into the pipet Never weigh hot chemicals or equipment When heating a test tube always use a test tube holder and be certain never to point the open end of the test tube toward yourself or another person Handling glass tubing or thermometers to insert glass tubing into a rubber stopper lubricate the glass tubing with a drop of glycerin hold the tubing in your hand close to the hole and keep all glass pieces wrapped in a towel while applying gentle pressure with a twisting motion To prepare a dilute acid solution from concentrated acid acid should be added slowly to water with continuous stirring This process is strongly exothermic and adding water to acid may result in a dangerous explosive spattering Use the fume hood for all procedures that involve poisonous or objectionable gases or vapors Never use an open ame and ammable liquids at the same time Keeping a Lab Notebook In keeping a lab notebook there are certain principles that should be followed These boil down to being clear and complete in your entries in your lab notebook There are also certain conventions for lab notebooks that are universally followed High on this list are the following Use a notebook with pre numbered pages Record entries in ink Keep entries reasonably neat and organized Never tear pages out of your lab notebook other than the carbonless copy pages What Kind of Notebook Should I Use For this class you must use a notebook with carbonless copy pages General Guidelines 0 Write your name on outside front of notebook Use black ink netipped ballpoint pen this will photocopy clearly At the front of the notebook leave a few pages for a Table of Contents Each lab should have a brief introduction and description of procedure Generally use only the right hand page for most text Use facing left page for working graphs manual calculation and working notes Prepare data tables in advance with columns for calculated results and notes Working graphs done in lab notebook to monitor progress Usage and Structure The overriding principle for a lab notebook is to record in it all the pertinent information about your lab work This boils down to clear descriptions of what you did and what you observed as a result It is a working tool and a reference for other researchers who might want to read your notebook and reproduce your work This applies to notebooks in learning laboratories Your lab instructor may want to look at what you did in order to understand your results This is often the case So it needs to be clear The word clear here is crucial In order to be clear data must be recorded in well thought out tables clearly labeled Descriptions of procedures must be clear and concise to the point You should record all your work in your lab notebook That is the proper place for all lab planning and observations Nothing should be recorded on odd scraps of paper etc Structure for your Lab Notebooks For each lab in this class you should have the following sections in your lab notebook Title Purpose Procedure and Observations It is also often helpful to include a Result section Note When preparing your notebook for lab only write on the right hand page Title With your lab notebook laid open on the right hand page write down the title of the experiment and the date In general you will use the righthand page for all your writing The lefthand page is reserved for recording scratch work Don t use this space until you need to One example of how to use the lefthand page if your work requires simple calculations using your measurements use the lefthand page to do the calculations If unexpected results occur later sometimes you can look back at your scratch work and discover the error Oh I subtracted wrong We put in 105 grams of copper sulfate not 95 like we thought Better to discover the error after the fact than never to discover it at all Purpose Below the title write the purpose of the experiment in one or two sentences This section serves to remind you and notify the reader what the experiment is about Procedure and Observations This next section will be labeled Procedure and Observations As the name suggests write down what you actually do and what you observe This section is where you should have pre prepared tables for data collection Set up this section by dividing the page into a right and left column In the left hand column write your procedure and in the right column next to the procedure record observations and data or measurements Results and Discussion You might want to include a final section that is labeled Results and Discussion In this section you would describe what results you got what conclusions you have reached ideas for continuing work etc 10 An example of a prepared notebook follows EXP NUMBER EXPERIMENTSUBJECT M DATE 7 02 o 4 NAME LAB PARTNER LOCKERIDESK NO COURSE amp SECTION N0 J quotquot 594 3 5 Ow 0 7 2Ce 7 9 o 7 To A74 7Z0 39 1r UK 0 Iquot394 IquotIrc 7 Le u 4 xx 6 1at 5 c n r n 7 i re R KL r max I I 0 V 0 m 0 can d 3 5 6 a ra 7om C1 C 39 C 0kr aIS39 TIA 4754quot Jdampr 539427 auL5 72 0ang or 744 mnzo g7Mae743 u 547 cwr ampz2rL7l4 7 Pl 0olt0LJ quot E D0 0EOlolta vcgtgt1f39o3 T LJe3 I00 v uooltc T 39 39os k 39 S o394r Z Je3 439 Z 9 So3a r39 W39u 0 Pb 3Tfav39Q r U 0 n 43Lalt L Me n quot aQlt 396xo hef quot5vv ox 0 S P v c39 71 9P Q0 T H4 CTN ov gtbY5 ovL uJar U0 Tlt39 6iAA wwkr Cc e T3L 7 ec o slt 39 5 uttr 39539amp ow4 Le T 3cps quot39l ngt Z 5uJ os w13L quot E PM oew WA c9 gto s quotT J J Ass uv fCSU3 vxs quotT I Fa s l S 6aJ o Tgrovs cf F39oslt s of 5 ur ggt04 5oJ39ov PL 33or39 quot5Sr JLquotf T 5 C r quot gt5t Ovx p L9r ml Fas 1 7 T C gt1V Z A Ras 3 5ox T 7 quot395k i T 44 SIGNATURE V DATE WITNESSITA DATE NOTE INSERT PERIODIC TABLE UNDER COPY SHEET BEFORE WRTTING 0 THE HAYDENMCNEIL STUDENT LAB NOTEBOOK 11 EXP NUMBER EXPERIMENTSUBJECT om 02of IIIAME LAB PARTNER LOCKERDESK N0 COURSE 8 SECTION NO o M A 00 10I 7Zo quotc39 quot4524 739 6zgtx 39 9 M3 oo ha c390 z 7 539 u la39 i A794 79ML 71 75624 4L wow quot0quot Ju5 L 14a7z W 3 SIGNATURE 39 DATE WITNESSITA DATE NOTE INSERT PERIODIC TABLE UNDER COPY SHEET BEFORE WRITING 0 THE HAYDENMcNEL STUDENT LAB NOTEBOOK 12 Writing Style in the Lab Notebook For certain entries in your lab notebook such as the Introduction before each experiment you should strive to write as logically and clearly as possible It is also a good idea to write in the third person passive voice to get into the habit and so that in many cases you can copy entries from your lab notebook into your reports without the need for major revisionsrewrite However this is a working document It is not expected that you write perfect prose in your notebook it is a first draft Just do the best you can Also as a working document with many entries being written while an experiment is in progress your observations it is understood that many entries will be brief but still record crucial observations Example Notebook entry Added 10 mL of 1M HCl solution tumed red instantly pcpti a few secs later gt clr soln When written into a lab report or journal article this would be expanded a bit and made grammatically correct l0 mL of 10 M HCl were added to the clear reaction mixture This immediately resulted in a crimson solution and a red precipitate formed a few seconds later leaving a clear solution Adapted courtesy of Keith James 13 Report Guidelines For most experiments performed this term you will turn in a type written report at the end of each lab you will find a summary of which sections to include in the report for that lab The reports are due at the beginning of class the week following completion of the experiment Below is a description of what should be included in each section The sections are presented here in the order they should appear in your lab report It is expected that you will complete each experiment and do the necessary calculations and analysis during the scheduled lab period each week You may discuss the calculations and analysis with your lab mates Your written lab report should be your own individual work The lab report sections should be complete but CONCISE For most experiments this term your report should be 12 pages long Writing Style You will write you reports using a formal scientific writing style A lab report must be written in the third person passive voice Also it must be in the past tense It should not contain personal pronouns such as I we or he neither should it contain proper names of persons Good 50 mL of 10 M HCl were poured into a 125 mL Erlenmeyer ask Bad I poured 50 mL of hydrochloric acid into a ask Also bad Joe Shmoe poured 50 mL of hydrochloric acid into a ask This is not the correct form of 3rd person It includes J oe s name Also bad We are going to put 50 mL of acid into the ask Uses future tense also we After you write your report there is one more thing to do before you print it and hand it in Proofread it Read it out loud If is doesn t sound right it isn t Fix it Then do it again until it is right You will enjoy writing reports more if you take pride in what you hand in Abstract This is like a condensed version of your lab report It is a standalone document Abstracts are in fact often published separately from the articles they describe A library search of the literature generally involves reading abstracts This is done with the aim to identify articles that need to be read in full and eliminate many others whose abstract makes it clear that they are not relevant to the study at hand So the abstract needs to be brief but complete There are three questions that should be answered in any good abstract 1 What did you do 2 How did you do it 3 What did you find Even though it sequentially appears first you should consider writing this part of the lab report after you have finished the remaining sections 14 Introduction Here you want to address WHY you did this experiment Your introduction begins with a statement of the purpose of the experiment You will do this again in the abstract but remember the Abstract is a stand alone document What you say there will not count you will find that as you write the report that you will be repeating yourself a bit Next provide any relevant background to put the experiment into context Include any key concepts mathematical equations or chemical equations needed by the reader to understand your experiment This means that your Introduction will often include some explanation of the theory behind the experiment Don t just write the equations but provide information as to why they are relevant You may consider writing your introduction with a central theme such as density types of chemical reactions Data This is section is where your experimental data belong In this section you would also include observations and descriptions of other pertinent events This section is not where the calculations interpretation and discussion of your results belong In published papers a data section is usually not included but this is a class so this section will be included Tables Whenever possible data should be presented in the clearest format possible usually in the form of a table When you present your data in a table it is necessary to take the following into account 0 Number tables sequentially as they appear Table 1 Table 2 Be sure to refer the reader to view the tables in the text Construct a descriptive table caption and place it above the table Tables should include descriptive column headings including units Tables should not be divided across page boundaries For a simple example see Table 1 Table 3 Mass of Water as Determined by a Fan Balance 001 g Here the volume of water delivered by a 10 mL volumetric pipet was determined utilizing the mass of water delivered and waters density 09980 gml Run mass water weighed g Volume water ml 1 995 2 994 3 998 Average 996 998 Error 002 002 Graphs When graphical presentation of data is necessary please prepare graphs using the following guidelines 0 Number gures sequentially as they appear Figure 1 Figure 2 0 In your writing be sure to cite the tables in the text 15 0 Insert a caption below he graph that indicates what is being plotted on the y axis vs what is being plotted on the x axis always y vs x 0 Each axis should be clearly labeled including units 0 Figures should not be divided across page boundaries 0 Remove gridlines titles and equations from the graph If this information is pertinent it should be included in the caption 0 If the slope or intercept is necessary for other parts of the experiment then place the values in the caption with proper units For a simple example see Figure 1 07 06 05 06 03 02 Absorbance at 512 nm 0 002 004 006 008 01 012 Concentration mm Figure 1 A calibration curve for the absorbance at 470 nm of aqueous Allura Red solutions as a function of the concentration A best fit line was rendered resulting in a slope of 586 mM391 Results The results section is where you should show sample calculations and report all of your results For every type of calculation you should show one sample calculation Each calculation should have a descriptive title ie Calculating the density of CocaCola The calculation section should be annotated The annotation is provided to describe why each calculation is useful and relevant to the lab activity The description should not be any longer than two or three sentences and should help you describe your results in your discussion section Sample calculations may be written by hand attached as an appendix to your report The results of all calculations should be summarized in a table where appropriate Calculating the density of CocaCola The volume 355 mL and mass 394 g of the contents of a can of coke had previously been determined above The density is determined utilizing the relationship dmv equation 1 which was explained in the introduction d 394 g355 mL 111 gmL Discussion In this section you will discuss interpretations of the experimental results This is where you get to present your thinking process For any labs that have questions to answer this is also where the answers get written up 16 The discussion is one of the most important parts of the lab report It is your chance to show WHAT YOUR RESULTS ARE and that you UNDERSTAND what you did in the lab This DOES NOT mean to include detailed procedures or that you need to reexplain your calculations in Words It DOES mean that a general description of the experiment can be useful in explaining your results and putting them in context In this section you should also discuss error analysis This does not necessarily mean trying to explain What Went wrong Maybe nothing did go Wrong It means discussing the limitations of your experiment For example if you are doing calorimetry in a coffee cup and the cup feels Warm to your hand it means that some heat is escaping Also if you are reading a 5 degree temperature change with a thermometer that you can only read to the nearest 05 degree there is a significant uncertainty in the exact magnitude of the temperature change You could easily have a 10 error or even more and this needs to be taken into account It at least needs to be mentioned to show that you were aware of the issue This is a limitation of the apparatus not an error on your part And yes if something did go Wrong your lab partner forgot to Write down the exact molarity of your reagent then that should go here too along with an explanation of how you attempted to correct for the error In this case you may have had to redo the experiment Adapted courtesy of Keith James 17 Example Lab Report Following is an example of a lab report prepared according to the previous report guidelines Sample calculations can be Written on a separate paper and attached to the report Calibration of a 10 mL Volumetric Pipette a 10 Abstract A 10 mL volumetric pipette was calibrated by determining the mass of Water delivered by a pipette A pipet was used to precisely deliver 10 mL of Water The mass of Water was then converted to volume using the density of Water The volume of the pipette was determined to be 998 002 mL when the mass of Water was determined on a pan balance and 9998 0002 mL when determined with an analytical balance Introduction A volumetric pipette is designed to deliver a stated volume of liquid however the actual amount of liquid any individual pipette delivers may vary slightly from this ideal stated volume In order to determine the actual volume an individual pipette delivers it will be calibrated In this case calibration refers to the comparison of the actual amount of liquid delivered by the pipette to the standard value of the pipette 10 ml Because delivered volume is being calculated another measurable quantity must be used to verify the volume delivered by the pipette In this case the relationship between mass and volume density will be used Mass is an easily measurable quantity that can be determined With a high degree of accuracy due to the availability of electronic balances Mass can then be converted to volume by the use of density D density m mass V volume The density of Water at a variety of temperatures is readily available and will be used here to calibrate the volume of the pipette Data Diameter of beaker 39 cm 01 cm Mass of Water evaporated in 60 seconds 00016g 00002g Temp of Water 205 C 02 C Density of Water 09980 gml Table 1 Mass Determined by Pan Balance 001g Run mass beaker g mass beaker Water g 1 2788 3783 2 2788 3782 3 2788 3786 Table 2 Mass Determined by Analytical Balance 00001g Run mass beaker g mass beaker water g ttransfer tWeigh 1 272349 365618 22900 23030 2 272348 367813 23200 23320 3 272335 368251 24120 24230 18 Results Calculation of the volume of water In this calculation the average mass of water for the three trials shown in Table 1 as determined by the pan balance was divided by the know density of water at 205 C The data are summarized in Table 3 Volume 996 g 09980 gmL 998 mL Table 3 Mass of Water as Determined by a Pan Balance 001g Here the volume of water delivered by a 10 mL volumetric pipet was determined utilizing the mass of water delivered and waters density 09980 gml Run mass water weighed g Volume water ml 1 995 2 994 3 998 Average 996 998 Error 002 002 Calculation for the mass evaporated To correct for evaporation of water in the time it takes to measure the mass of the water delivered by the volumetric pipet the mass of water that evaporated was estimated The rate of evaporation of water in the 50 mL beaker in 60 seconds 00016g The data are summarized in Table 4 Mass evaporated rate of evaporation X time of evaporation 00016 g 60 s X 90 s 00024 g Calculation of the mass transferred The mass of water initially transferred was the sum of the mass of water evaporated and the mass of water present at the time of weighing Table 2 The data are summarized in Table 4 Mass transferred mass water weighed mass transferred 99769 00024 99793 g Table 4 Mass of Water as Determined by an Analytical Balance 00001 g Here the volume of water delivered by a 10 mL volumetric pipet was determined utilizing the mass of water delivered and the density water 09980 gml A correction was added to account for the water that evaporated during the measurement Run mass water weighed g tevap s mass evap g mass transferred g Volume ml 1 99769 90 00024 99793 2 99735 80 00021 99757 3 99756 70 00019 99775 Average 9978 9998 Error 0002 0002 Discussion The mass of water delivered by a 10 mL volumetric pipette was determined on both a pan balance and an analytical balance Tables 1 and 2 respectively The mass of water was then 19 converted to volume using the density of Water In the case of the analytical balance the rate of evaporation of Water Which is a systematic error was taken into consideration In this case the mass of Water that evaporated from the time the Water was delivered to the beaker to the time of Weighing was added to the Weighed mass of Water delivered by the pipette This correction was not necessary when the pan balance was used since the accuracy of the pan balance is 001 g and the evaporation rate of Water under experimental conditions was found to be 27 x 10395 gs The use of the analytical balance increased both the precision and the accuracy of the calculated volume of the pipette 998 002 mL with the pan balance and 9998 0002 mL with the analytical balance see tables 3 and 4 The improvement in the results can easily be seen by the percent error which was calculated to be 02 with the pan balance and 002 with the analytical balance The largest source of error in this experiment most likely came from the difficulty in accurately filling the pipette to the mark with Water which introduced random error into the experiment 20 General Chemistry Lab Report Checklist General Have you listed your name partner39s name a descriptive lab title and date Did you use spellchecker Is your report Written in passive third person voice you did not use the Words I We they etc Is proper tense is maintained Within sections Have you correctly Written your chemical formula and names correctly Were correct subscripts superscripts and symbols are used Did you separate the numbers from their units 025 mL was added not 025mL was added j Did you check significant figures Do your numbers include leading zeros 025 mL was added not 25 mL was added Did you make sure that you did not start a sentence with a number Are your references cited in one official style Have you made a citations Whenever ideas from outside All subjects and verbs are in agreement Did you make sure that there are no run on sentences or fragments Abstract The abstract is a condensed summary of the report39s findings Abstracts are often Written last They should be clear concise and self contained and in the context of this lab approximately three sentences long What did you do Identify the rationale behind the investigation How did you do it summarize the procedure Without using specific steps Present the important findings numerically including error statistics Introduction The introduction will provide the reader information on What you are doing Why you did it and critical background information necessary in understanding the methods and results of your experiment Did you include a statement of purpose Is there sufficient background so that the reader can understand what you did Are necessary equations chemical or mathematical included Data This section should give only the data and observations from the lab Without results Are your data tables properly formatted Are your calculations either attached as an appendix or typed neatly into the data section Are your figures and tables numbered sequentially and referred to in the text Table captions above and figure captions below Tables and figures are not broken over multiple pages Are the axes on your graphs formatted properly with labels Are all graphs and tables accompanied by a Written description relating the same 21 information to the reader Results We will be treating this section as a calculational section This is Where you will be showing all calculations along with a written description as to how the calculations were carried out and What the result of the calculation is and how it relates to the lab Your readers must easily find your results in order to evaluate and interpret them Are calculations accompanied by text explaining the both the method of calculation and results of the calculation jUnits Significant Figures jls a straight forward presentation of the results of your experiment included in either a table or in text jCan your key results be understood by a reader Without reliance on figures and tables Discussion In this section you will discuss interpretations of the experimental results It will be necessary to describe your results cite tables or figures It should include a general description of the experiment to put the results into context Can your key results and discussion be understood by a reader Without reliance on figures and tables jAre key results highlighted and carefully explained jDid you make logical deductions based on the results usually questions are given in the lab manual to help this jHave you discussed sources of error or ambiguities in the data jDid you confirm all relationships that were stated in purpose or abstract jDo your conclusions clearly contribute to the understanding of the overall problem 22 CH227 LABS PreLab Who has the same solid that I have Part A Answer the following questions in your lab notebook be sure to show work for any calculations 1 Some of the chemicals you will use this year are hazardous One way of determining the risk of using chemicals is to read the Material Safety Data Sheet MSDS These documents provide a wealth of information regarding the safety risks of each compound Do a web search with the key words MSDS and Lead Nitrate Read through the MSDS and determine the steps that need to be taken in case of accidental skin exposure your most common risk in the lab Look up the MSDS for both Hydrochloric Acid and Sodium Hydroxide What steps need to be taken if there is skin exposure Note particularly the danger of getting NaOH in your eyes and be sure to wear goggles at all times in the lab In this lab there are many possible unknown compounds including ammonium iodide sodium acetate silver nitrate calcium carbonate lithium carbonate aluminum chloride and potassium iodide Pick one of the listed compounds and read the MSDS on it Additionally search the internet for two interesting factoids regarding the chemical compound you choose Part B Prepare your lab notebook for the experiment This includes stating the purpose of the experiment and summarizing the procedure in a bulleted list format be sure to include space for observations At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 23 quotWho has the same solid that I havequot Science is generally a cooperative collaborative affair most discoveries are not made by just one person It is important for scientists to be able to communicate their data and be prepared to share data or samples In this experiment you will be trying to determine who else in your lab section has the same unknown compound as you This will be done while learning some basic techniques that are used for the analysis of chemical compounds You will see as you progress throughout the year in chemistry that compounds can be classified in many different ways ionic or molecular acidic or basic metals or nonmetals Think about how you might classify water on the basis of easily observable properties We know that water is clear colorless freezes at 0 C boils at 100 C dissolves most salts has a density of 1 gmL and is composed entirely of hydrogen and oxygen atoms in a definite ratio this list could go on and on To enable classification it is important to be able to accurately determine describe and compare the chemical and physical properties of compounds You will be given a sample of an inorganic solid and will determine your sample s properties such as the solid s relative solubility its relative melting point the electrical conductivity of the substance and its aqueous solution the aciditybasicity of the compound s aqueous solution its appearance in a ame and its reactivity Your goal is to identify other students in class who have the same compound that you have Comparisons of different samples may be made by doing sidebyside analysis using the same techniques Guided by your TA your lab section will determine a method for sharing reporting your observations You should identify the people in your lab section that had the same substance and then run some confirmatory tests to verify that the solids are the same This is a list of some of the physical and chemical properties that you will investigate during this lab exercise 1 Melting points A substance s melting point temperature will depend on the bonding type or intramolecular forces in the sample Some compounds have melting points greater than 200 C while others have lower melting points Upon further heating some compounds may decompose into simpler compounds or burn 2 Conductivity of aqueous solutions When dissolved in water some compounds dissociate into ions These dissolved ions move through the solution and thus conduct electricity 3 Crystalline or amorphic As a result of the types of bonds in the compound some substances form very regularly shaped crystals Others are less able to form regular patterns so their solids are less geometric Crystalline compounds are hard and brittle because the ions are locked tightly into place by their electronic interactions As aresult it s di icult to move these ions apart and when they do move apart the whole crystal typically breaks 4 Flame test Some atoms emit characteristic visible colors when excited by an energy source like a ame This colored light is a characteristic signature of the element which is a consequence of the electronic structure of the element These emitted colors can be used to 24 identify the elemental composition of a substance For instance potassium produces a violet color While lithium will emit a vibrant red 5 Acidic basic or neutral aqueous solutions Some substances will make a solution acidic or basic when they dissociate into ions when dissolved in Water Some ions have the ability to act as acids in solution while others act as bases When a substance is dissolved in Water these properties can easily be tested using pH paper 6 Reactivities Each compound has a characteristic reactivity that may or may not be easily elucidated By mixing an aqueous solution of the unknown with an aqueous solution containing another compound reactivity pattems may become visible Reactions are usually visualized by looking for the formation of a solid gas or change in color This lab is based upon the journal article quotWho Has the Same Substance that I Havequot A Blueprint for Collaborative Learning Activities Brian P Coppola Richard G Lawton Journal of Chemical Education 1995 72 12 1120 and Identification of Ionic and Molecular Compounds httptinvurlcom3if6oq6 np nd Web 24 Aug 2011 25 MATERIALS Bunsen burner striker multiwell microplate magnifying glass and conductivity meter all available in the lab room PROCEDURES 1 Physical characteristics a Obtain a small peasized sample of your assigned unknown b Using a magnifying glass examine the sample and record your observations in your lab notebook 2 Determine conductivity in the solid state a Using the sample obtained previously test for electrical conductivity using the conductivity meter supplied by touching the probes to the sample Be sure the probes are dry b Record your observations 3 Determine the relative solubility of each unknown a Add 12 of your pea sized sample of the unknown to a small test tube Add 2 cm or approximately 1 fingers width of deionized water to the test tube b Mix with your microspatula c Record your observations as S soluble or IN insoluble soluble means that a clear solution has formed insoluble means that the sample is cloudy or that there is undissolved solid left in the test tube d Do not discard the contents of the test tube 4 Determine the conductivity of the solution made in step 3 a As a control use the conductivity meter to check the conductivity of deionized water and record your observations b Pour a little of your unknown aqueous solution into one well of the microplate test for conductivity and Rrecord your observations c Do not discard the contents of the microplate 5 Determine if the aqueous solution is acidic basic or neutral a Dip your microspatula in your solution and wipe it on a piece of pH paper The paper is normally orange It will turn red if the solution is acidic or blue if basic b Record your observations 6 Determine the reactivity of your unknown a Using a dropper equally divide your solution amongst three wells including the one you already used in your multiwell microplate The wells should not be full b Add 5 drops of 1M hydrochloric acid HCl to the first well The symbol M represents molarity a unit of concentration The greater the molarity the greater the concentration c Add 5 drops of 01 M leadII nitrate PbNO32 to the second well d Add 5 drops of 10 M sodium hydroxide NaOH to the third well Waft a water moistened piece of pH paper over the third well to see if a gas ammonia is 26 6 f produced Note aqueous solutions of ammonia are basic and the pH paper would turn blue in the presence of ammonia gas Record your observations such as tumed cloudy or no change Clean your test tube multiwell plate and dropper as directed by your TA 7 Flame test done in the fume hood a C Dip a Nichrome wire into a beaker of water and place it in the hot part the blue inner cone of the ame until it glows redorange Do at least 5 times or until the color of the ame is consistent This cleaning step removes old samples from the wire Dip the wire into the previously unused half of your unknown solid to pick up a small quantity of the solid Place the wire with the solid on it in the hot part of the ame Observe the color of the ame Note some substances will not show a positive ame test no color change Record your results 8 Determine the relative melting point for each unknown compound done in the fume hood a b 6 f Carefully light the gas burner unless it is still burning from the ame tests Using a scoopula obtain a small portion 12 of the amount remaining from step 1 of your unknown solid Carefully heat the sample on the edge of the scoopula 1 inch from the very top of the ame not the hottest part of the ame Monitor the time required to melt Any substance that will melt under normal lab conditions will do so quickly don t heat any substance longer than 20 seconds Record your observations based upon whether or not the substance melted and how long it took remember heat no longer than 20 s Clean the scoopula as directed by your TA 9 Record your data as indicated by your TA and identify all people in the lab section having the same substance based on your collective observations 10 Run some confirmatory tests to verify that you have the same compound as the other groups Choose E test that makes your unknown unique from the other substances and do a sidebyside comparison to verify your conclusion 11 Clean up DATA All remaining solutions and solids must be placed in the properly labeled Waste jar Your lab area should be wiped clean and all glassware and equipment should be placed in your lab drawer Guided by your TA you will construct a table to report your results and observations For the TA The most important practical aspect of setting up this experiment is to ensure that the identi cation is based on the experimental data that are collected by the students Please discuss contamination and how to avoid contaminating the stock solutions and unknowns 27 Possible unknowns include ammonium iodide sodium acetate silver nitrate calcium carbonate sugar lithium carbonate aluminum chloride citric acid potassium iodide some of which are hazardous chemicals 28 Who has the same solid that I have Lab Report Your report for this lab should include the following sections Abstract Your first draft abstract for this lab should be written as part of a postlab discussion led by your TA You will refine this later on your own Introduction Describe what you did tested and compared several unknown compounds and provide a bit of insight as to what techniques were used Explain why you did this experiment to match the properties of your unknown with other unknown substances in the class Data Include your data tables Pay attention to the directions above about formatting tables Be sure to include your unknown and the data from the confirmatory test Results Write a paragraph explaining the results of this experiment I Give a brief summary of the results obtained for your unknown and the matching unknowns specifically list which results were similar and which results were different I Give a brief summary of the results of your confirmatory test specifically list what the test was why you picked the particular test you did and your observations from the sidebyside experiment I Be sure to indicate which unknown number you tested and the matching unknown numbers Discussion Write a paragraph that discusses the following points I Discuss how you matched your unknown sample with the others in your lab section Indicate how your results for your solid caused you to identify its match How did your solid s properties differ from the others I Provide some error analysis For instance what sort of weaknesses do you see in the procedures or the way the data were reported that may have caused some ambiguity 29 PreLab How much sugar is in a can of coke Part A Answer the following questions in your lab notebook be sure to show work for any calculations 1 A solution has a mass of 1095 g and a volume of 1000 ml what is the density of the solution 2 An object has a density of 025 gml would you expect this object to oat on water 3 How many milliliters are in 1 pint 4 What is the purpose of constructing a calibration curve If you re not sure you might want to watch the weblet online presentation for this lab before you answer Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 30 How Much Sugar is in a Can of Coke GOALS 1 Determine the amount of sugar in grams in a can of coke 2 Learn how to make solutions quantitatively 3 Learn how to make and use a calibration curve INTRODUCTION If you were to measure out identical volumes of Coke and diet Coke you would find that the two liquids have different masses This difference in the mass of the two liquids is best discussed by looking at the mass per unit volume or density of the two liquids M Density Mass Volume D V Density is a convenient quantity because it is independent of the volume used scientists describe properties like this as intensive Intensive properties like density are independent of the amount of substance and thus the density of two different solutions can be compared without needing to have the same volume of the two solutions When comparing Coke and diet Coke it is found that Coke is more dense than its sugar free relative To understand why a molecular view of the two substances is useful The main difference between the two solutions is the presence of the dissolved sugar in Coke that is absent in diet Coke The sugar makes Coke more dense than diet Coke To a first approximation Coke can be represented as a solution of sugar dissolved in water As the amount of sugar dissolved in a given volume of water increases so does the density of the resulting solution This makes it possible to determine the mass of sugar in Coke by comparing it to solutions with known concentrations of sugar The relationship between the amount of dissolved sugar and the density of sugar water solutions will be determined using a calibration curve Calibration curves are constructed using known quantities called standards Calibration curves allow you to determine the content of an unknown by comparing it to observations made on the standards with known values of the property being measured In this case you will prepare standard solutions of known volume with a known amount of dissolved sugar After obtaining the mass of these standards using an instrument called a balance and calculating the density of each solution you will prepare a graph of density vs mass of dissolved sugar You will then determine the mathematical relationship between the two quantities Once the relationship between density and sugar content is determined you will use this relationship to determine the amount of sugar in Coke SCIENTIFIC GRAPHS This experiment will also serve to introduce you to scientific graphing Here we will introduce what must be included in any scientific graph Whenever you are asked to produce a graph from laboratory data either by hand or using a computer program all of the following criteria must be met 1 All graphs should have a title except when included in a report or other scientific writing in which case you substitute a figure caption below the figure for the title 2 Both axes must be labeled with a name and units 3 The graphed data must take up the full space of the graph 31 P The minimum size of a graph should be 12 of a standard sheet of paper When a best fit line to the data is computed and used the line and equation if applicable should be shown on the graph 6 The independent Variable is the xaxis and the dependent Variable is the yaxis and the graph is referred to as dependent VS independent for example the graph below is Mass in grams Vs Volume in mL Figure 1 shows an example of an acceptable scientific graph of raw data Figure 2 demonstrates the proper way to represent a linear fit on a graph 9 Figure 1 The Relationship Between Mass and Volume for Water 50 9 40 9 3 8 30 c U E 20 9 10 9 O I I I I I O 10 20 30 40 50 60 Volume ml Figure 2 The Relationship Between Mass and Volume for Water y 10015x 0009 50 40 3 30 U E V 20 V 10 V O 10 20 30 40 50 60 Volume ml Graphing Using Microsoft Excel An excellent tutorial on graphing with MSExcel can be found at the following website httpwwwncsuedulabwriteresgtgtmenuhtml 32 This is a list of the basic steps necessary to graph data and do a linear regression the generation of a best t line using Excel Basic Graphing l 2 3 U With the program open enter the data to be graphed in the cells Enter X data in one column followed by y data in an adjacent column Click and drag the mouse to highlight all the data to be graphed Click on the chart Wizard icon Choose XY Scatter for the chart type and the unconnected points icon for the Chart sub type Click next A preview of your chart will appear If it appears correctly click next Enter a chart title and the axis labels and click finish With the chart selected you can also access the title and axis labels by selecting Chart then chart options from the drop down menu Adding a Linear Trendline to a Graph 1 2 3 4 With the graph selected select Chart then add trendline from the drop down menu Select linear as your regression type Select the options tab in the popup window Select the display equation on chart button and click ok 33 PROCEDURE Calibration Curve You will make five sugar water solutions to start Each solution should have a different amount of dissolved sugar covering a range from about 1 8 g of sugar per 50 mL of solution volume To make the solutions in a quantitative manner they must be prepared in volumetric asks Volumetric asks are designed to accurately contain a specific volume Volumetric asks are marked with a fill line When filled to the marked line the ask accurately holds the stated volume these devices are called TC for to contain When putting the last bit of solvent into volumetric asks it is best to bring the uid to the line carefully by using a wash bottle or eyedropper to assure that the ask is not overfilled causing you to have to start over To accurately know the mass of sugar mL used in each of your five standard solutions follow these steps 1 Weigh the empty ask with the stopper and record the mass 2 Weigh out the desired mass of sugar in a weigh boat This mass does not need to be recorded 3 Add the sugar to the ask Weigh the ask containing the sugar with the stopper and record the mass The difference between the two masses is the mass of sugar 4 Add water to the ask until it is approximately half way to the fill line Swirl the ask to dissolve the sugar Do not shake the ask 5 Once the sugar has completely dissolved add water carefully to the fill line Put the stopper in the ask and invert it ten times to ensure that the solution is thoroughly mixed The bubble should run all the way down the neck of the ask to the stopper each time you invert the ask 6 Weigh the capped ask containing the solution and record the mass The difference between this mass and the first one is the mass of the solution the volume is 50 ml if you carefully followed these instructions As noted above the mass of sugar used for each solution is found by subtracting the mass of the empty stoppered ask from the mass of the stoppered ask containing sugar The mass of the solution is found by subtracting the mass of the empty stoppered ask from the mass of the stoppered ask containing the solution Below is an example of an acceptable table to present the data from this experiment In this experiment a table is supplied for you In later experiments you will be expected to produce your own data tables for your notebook and the Results section of your lab reports Mass of Mass of Mass of Mass of Mass of Density Dissolved Empty Flask Flask Sugar Solution of sugar per Flask Stopper Stopper g g Solution mL Stopper With With gml solution g sugar g solution g gml Flask 1 Flask 2 Flask 3 Flask 4 Flask 5 34 Using the data in the above table construct a graph of density of solution y vs dissolved mass of sugar per mL of solution X and fit the data to a linear relationship as described above Report the equation for the line on the graph Graphs must be prepared using the computer Your TA will assist you with this if needed This graph represents the relationship between the density of the sugar water solution something that can be measured and the amount of dissolved sugar in the solution something that cannot be measured directly but could be controlled in making the standards Determine the amount of sugar in a can of Coke Weigh and record the mass of a dry clean 50 mL volumetric ask before carefully filling the ask to the fill line with the at Coke provided Weigh and record the mass of the ask containing Coke Determine the density of the Coke Put the used Coke in the provided waste jar RESULTS When a linear relationship exists between two quantities density and amount of sugar it is only necessary to measure one of the quantities density and know the relationship found from your calibration curve before the other quantity amount of sugar can be determined By finding the density of coke yaxis and drawing a line to your calibration curve then drawing a vertical line down to the Xaxis mass of sugar per mL solution you can graphically determine the amount of sugar dissolved in each mL of Coke Altematively you can invert the relationship given by the calibration line equation to solve for X from the observed density y Both methods should give the same result for mass of sugar per mL of Coke In order to find the mass of sugar in one can of Coke you will need to consider the volume of a can of Coke 12 ounces One liter contains 338 uid ounces Calculate the percent error in your determined value based on nutritional information given on the label on a can of Coke 35 Sugar in Coke Lab Report Your report for this lab should include the following sections Abstract Your first draft abstract for this lab should be Written as part of a postlab discussion led by your TA Data Include your data table Results Include a copy of your calibration curve Report the amount of sugar in 1 mL and 1 can of coke Present the percent error for your amount of sugar in a can of coke with respect to the number given on the label Attach hand written sample calculations to the back of your report Discussion Discuss the experiment and any possible sources of error 36 PreLab A Cycle of Copper Reactions Part A Answer the following questions in your lab notebook be sure to show your work for any calculations 1 In chapter 4 of your text read about the different types of reactions What causes a precipitate to form when certain combinations of aqueous salt solutions are mixed 2 Locate the solubility table in chapter 4 and summarize which ions are generally soluble and which are generally insoluble 3 Look up the MSDS for nitrogen dioxide gas What dangers does it present and what steps need to be taken to avoid exposure 4 This is one of the most dangerous experiments during this term because of the risk of exposure to dangerous chemical substances For example concentrated nitric acid is a particularly nasty solution what happens when it comes in Contact with skin What safety precautions need to be taken to avoid exposure All of the acids and bases used in this experiment are also potentially dangerous and should all be handled carefully Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 37 A Cycle of Copper Reactions GOALS 1 Cycle solid copper through a series of chemical forms via aqueous phase reactions 2 Learn about and identify different types of aqueous reaction types 3 Calculate percent recovered copper after all of the transformations INTRODUCTION This experiment will cycle elemental copper through a series of five reactions summarized below CuNO32 NaOH gt CuOH HNO3 Cu heat Zn Cl Cuso4lt H280 C110 The cycle will both begin and end with pure elemental copper At different stages of the cycle copper will be present in different chemical forms At times copper will be present in solid compounds and other times in ionic form Each chemical change that copper undergoes is observable as a change in the physical properties of the solution or precipitate As you perform each reaction be certain to observe and record your observations of all physical changes At this point in the term you should have been introduced in the lecture class to three different types of aqueous reactions precipitation reactions acid base reactions and oxidation reduction or redox reactions in addition the text may have discussed gas forming reactions In precipitation reactions soluble cations and anions combine to form an insoluble compound that leaves the solution as a solid precipitate In acid base reactions an acid and base react to produce water and a salt Redox reactions involve the transfer of electrons As you go through the series of reactions you should be able to classify each reaction with the exception of reaction 3 as one of the three above described types of aqueous reactions Reaction 1 The first reaction proceeds according to the following balanced chemical equa on 4 HNO3 aq Cu s gt CuNO32 aq 2 H20 1 2 N02 g 38 In this first reaction elemental copper metal is reacted with concentrated aqueous nitric acid solution The result of this reaction is that copper changes from its elemental state charge 0 to an aqueous ionic state Cu2 in an oxidation reduction reaction Reaction 2 The second reaction then converts the aqueous Cu2 into the solid copper II hydroxide CuOH2 through a precipitation reaction with sodium hydroxide according to the following balanced chemical equation CuNO32 aq 2 NaOH aq gt CuOH2 s 2 NaNO3 aq Reaction 3 The third reaction takes advantage of the fact that CuOH2 is thermally unstable When heated CuOH2 decomposes breaks down into smaller molecules into copper II oxide and water according to the following decomposition reaction equation CuOH2 s heat gt CuO s H20 1 Reaction 4 When solid CuO is reacted with sulfuric acid the copper is retumed to solution as an ion Cu2 according to the following acid base metathesis double displacement reaction equation CuO s H2SO4 aq gt CuSO4 aq H20 1 Reaction 5 The cycle of reactions is completed in this reaction where elemental copper is regenerated according to the following oxidation reduction reaction equation Thhis reaction changes copper from its ionic state Cu2 to its elemental state by exchanging electrons between zinc and copper CuSO4 aq Zn s gt ZnSO4 aq Cu s Here zinc and copper exchange physical and oxidation states in and out of acidic solution Hydrochloric acid is then used to dissolve any excess zinc The solid copper can then be collected washed dried and weighed Some copper is bound to be lost in all of the chemical transformations so the percent recovery mass copper remaininginitial mass x 100 is expected to be less than 100 39 PROCEDURE Be sure to discard all waste in the waste jars as directed by your TA Reaction 1 Caution Concentrated nitric acid is hazardous Avoid getting it on your skin or clothing If you do get any on your skin or clothing wash it off immediately with running cold water Do not breathe vapors Weigh out about 05 g of copper Be sure to record the actual amount used to the nearest milligram Place the copper at the bottom of a 250 mL Erlenmeyer ask In a graduated cylinder carefully measure out 50 mL of concentrated nitric acid DO THIS NEXT STEP IN THE HOOD N02 gas is toxic In the fume hood add the nitric acid to the ask containing the copper The nitric acid should completely cover the copper Be sure to record all observations Remaining in the hood swirl the ask until all of the copper has dissolved Once the reaction is complete and the gas has dissipated add 20 mL of DI water to the ask Once you are sure that all the gas has been removed in the fume hood you may return to your workbench Reaction 2 While stirring with a glass rod slowly add 20 mL of 60 M NaOH to the ask Be sure to record all observations Reaction 3 With occasional stirring slowly heat the ask on a hot plate until the solution just begins to boil At this point you should notice that the blue CuOH2 has been converted to black CuO If the conversion does not appear to be complete not all of the blue CuOH2 has disappeared heat the ask a little longer andor ask your TA to take a look Do not let the solution boil vigorously Once the conversion is complete remove the ask from the hot plate and allow the CuO to settle In a clean beaker heat 200 mL of distilled water Add 50 mL of nearly boiling hot water to your reaction mixture Once the CuO has resettled give it about 5 minutes to settle carefully decant the supernatant liquid removing as much as possible without losing the desired product CuO Be sure to record all observations Reaction 4 Carefully add 5 mL of 60 M HQSO4 sulfuric acid to the ask and swirl the mixture for 1 minute All of the black CuO should dissolve and be gone at this point If there is still black solid in your reaction mixture add an additional 1 mL aliquot of the sulfuric acid and swirl the mixture for an additional minute If the black solid has still not dissolved ask your TA or repeat the addition of 1 mL of sulfuric acid as necessary Be sure to record all observations 40 Reaction 5 Caution Concentrated hydrochloric acid is hazardous Avoid getting it on your skin or clothing If you do get any on your skin or clothing wash it off immediately with water Do not breathe vapors DO THIS NEXT STEP IN THE HOOD Hydrogen gas is generated which is extremely ammable There should be no open ame in the room Additionally there is some possibility of producing more nitrogen dioxide gas so do not breathe the vapors Add all at once 10 g of 30 mesh zinc or zinc powder Stir until the supernatant liquid is colorless Decant the supernatant liquid Remain in the hood and add 5 mL of distilled water followed by 10 mL of concentrated hydrochloric acid The hydrochloric acid removes any excess zinc according to the following balanced chemical equation Zn s 2 HCl aq gt ZnCl2 aq H2 g If the hydrogen gas evolution stops before all of the solid zinc has been removed more acid in 1 mL aliquots can be added Once the evolution of hydrogen gas has become very slow the ask may be returned to the workbench You may warm the mixture on a hot plate to speed up the reaction but do not boil the solution Once the hydrogen gas evolution has completely stopped remove the ask from heat and carefully decant the liquid Transfer the solid copper to a clean pre weighed beaker Using a wash bottle to wash the copper metal into the dish can facilitate the transfer Wash the copper at least twice with about 5 mL of distilled water each time Decant the water after each wash Wash the copper with an additional 5 mL of methanol Allow the copper to settle and decant the methanol Gently heat the copper on a hot plate to evaporate any remaining methanol and dry the copper Once dry remove the copper from the hot plate and allow the beaker to cool before determining the mass of the recovered copper Be sure to record all observations and the final mass of the copper RESULTS Once the mass of recovered copper is known difference between the pre weighed beaker plus copper and beaker alone the percent recovery can be calculated from the following formula Percent recovery mass of copper recoveredinitial mass of copper100 REFERENCE 1 Condike GF J Chem Ed 1975 52 615 41 Copper Cycle Lab Report Your report for this lab should include the following sections Abstract Your third abstract in this class will be written individually Keep it to three sentences and be sure to discuss what you did how you did it and what your results percent recovery were Introduction Data Begin with a statement of the purpose of the experiment observe various reactions involving copper Provide any relevant background and key concepts there are different types of chemical reactions Include useful chemical equations include each balanced equation and what type of reaction it is Include each balanced chemical reaction and your observations for each reaction Include the initial and final mass of copper Results Report the observations that suggested that a chemical reaction took place in each step Report your percent recovery of copper Attach hand written sample calculations to the back of your report Discussion Discuss the experiment reaction types involving copper Discuss your percent recovery and possible sources for loss of copper during the reaction cycle Here are some things you can think about 0 Did any of your discarded waste have a blue color if so what does the blue represent and how would it change your recovery 0 Did you lose any black solid while decanting and if so what were you discarding and how would it change your recovery 0 Was there any solid Zn remaining at the end and if so how would that change your recovery 0 Was your copper completely dry when you massed it at the end of the lab and how would it change your recovery 0 Note that some of the items above increase calculated recovery and some decrease it identify the direction of the error in each case 42 Prelab Which Alkali Metal Carbonate Part A Answer the following questions in your lab notebook be sure to show your work for any calculations 1 What is the law of conservation of mass 2 How many moles of CO2can be produced by the complete reaction of 153 g of lithium carbonate with excess hydrochloric acid balanced chemical reaction is given below 3 In the synthesis of barium carbonate from an alkali metal carbonate MZCO3 where M is one of the alkali metals a student generated 3723 g of barium carbonate from 2001 g of their alkali metal carbonate The reactants were MZCO3 and barium chloride Write the balanced chemical reaction for this synthesis 4 How many moles of barium carbonate were produced 5 How many moles of alkali metal carbonate were reacted 6 What is the molar mass of the alkali metal carbonate Hint remember the units ofmolar mass are gmol 7 The chemical formula for the alkali metal carbonate is MZCO3 what is the molar mass of M Which alkali metal is closest to this molar mass 8 Look up the MSDS for barium chloride How toxic is this substance What steps need to be taken if there is skin exposure or accidental ingestion Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 43 Which Alkali Metal Carbonate The Problem In a search for a good cleaning formulation as in laundry detergent or a degreaser for metal parts alkali metal carbonates are found to be useful In natural deposits these carbonates may occur as crystals of a single alkali metal carbonate such as lithium carbonate or as amorphous solids with several of the alkali metal carbonates co deposited Imagine that you are an analytical chemist and have received a sample of a pure alkali metal carbonate from a newly discovered deposit Your task is to determine which alkali metal carbonate composes the sample You will do an experiment to determine the atomic Weight for the alkali metal in the carbonate you have and thus which alkali metal is present You will also evaluate sources of error as you compare your experimental values with the expected value for the atomic Weight of the alkali metal You will use two different methods to ensure confidence in your results Based on your experience you will be able to recommend which procedure you would use if you had time and resources for only one technique At the end of this experiment you will prepare a report giving your experimental results This will include the identification of your alkali metal carbonate error discussion for both methods and a rationale for the preferred method you would recommend to the lab 44 Gravimetric analysis Method 1 In this part of the experiment you will perform a synthesis and use reaction stoichiometry to identify your unknown alkali metal carbonate The reaction involves your aqueous carbonate reacting with barium chloride BaCl2 in a precipitation reaction The product is an insoluble barium carbonate You will isolate and weigh it PROCEDURE Be sure to discard all waste in the waste jars as directed by your TA 1 Add a 10 g approximately but massed to mg accuracy and recorded in your notebook sample of your carbonate to a 250 mL beaker Add 50 mL of water and stir until the carbonate is completely dissolved To precipitate the barium carbonate add 200 mL of 10 M BaCl2 solution to the sample and stir until wellmixed CAUTION Use gloves to handle the barium compounds Heat the BaCO3 formed to digest the precipitate causing the precipitate to form larger aggregates This involves boiling the solution for 5 minutes with little agitation Weigh two pieces of filter paper Be sure to record the masses in your notebook Filter the barium carbonate using both pieces of filter paper stacked in a Buchner funnel using vacuum filtration as demonstrated by your TA Wash the precipitate with water using the vacuum to pull the water through the filter Wash the precipitate with a small amount of ethanol Carefully remove the solid and filter paper from the funnel and place your product on a pre weighed watch glass Allow the solid to dry until near the end of the lab period It may be necessary to put the watch glass on a hot plate on LOW heat to speed up the drying process CAUTION The filtrate solution left after filtration to isolate barium carbonate contains excess Ba2 Dispose of in the proper waste container see TA if you are unsure of the proper procedure DO NOT DUMP THIS SOLUTION DOWN THE SINK Weigh the dry solid and record the mass in your notebook Calculate the mass of barium carbonate by difference removing mass of watch glass and filters Analysis Use the following questions to lead you to the identity of M Determine the mass of barium carbonate produced as above How many moles of barium carbonate is this Use stoichiometry to determine how many moles of M2CO3 reacted to produce the barium carbonate and hence were present in the 100 g sample you started with Determine the molar mass of the unknown metal carbonate and compare it to the molar mass of the possible alkali metal carbonates Li2CO3 Na2CO3 K2CO3 What alkali metal is M most likely to be 45 Where are the errors most likely to enter into the experiment Simple weight loss Method 2 In this experiment you will make use of the principle of conservation of mass to determine the identity of your alkali metal carbonate The metal carbonate a base will react with added acid to produce carbon dioxide CO2 gas which will leave the system and go into the gasphase Applying the law of conservation of mass you can determine the mass of CO2 evolved by difference from the starting mass of the reagents The balanced chemical equation will allow you to determine the atomic mass of the alkali metal M2CO3 2 HC1 6 CO2 2 MC1 H20 Use the procedure outlined below to help you design your experiment and set up your data table Answer any questions you encounter along the way You should perform the procedure three times and should obtain a relative deviation largest result smallest resultaverage result x 100 of less than 10 PROCEDURE 1 Place 10 g of your unknown in a pre weighed 250 mL beaker 2 Measure 400 mL of 1 M HC1 in a pre weighed graduated cylinder Determine the actual mass of HC1 added Pour the HC1 slowly onto the unknown metal carbonate 3 Measure the mass of the beaker after the reaction has ceased no further generation of carbon dioxide gas Analysis Use the following questions to lead you to the identity of M Determine the mass of CO2 produced How many moles of CO2 were produced Use stoichiometry to determine how many moles of M2CO3 were present in the 10 g sample you started with Determine the molar mass of the unknown metal carbonate and compare it to the molar mass of the possible alkali metal carbonates Li2CO3 Na2CO3 K2CO3 What alkali metal is M most likely to be Where are errors most likely to enter into the experiment Which direction do these errors bias the final answer molar mass of metal carbonate Experiment adapted from Dudek E P J Chem Educ 1991 68948 46 Which Alkali Metal Carbonate Lab Report Your report for this experiment should include the following sections Abstract Keep it to three sentences and be sure to discuss what you did how you did it and what your results were molar mass and which metal carbonate Introduction Data Begin with a statement of the purpose of the experiment using two methods to identify the alkali metal in the unknown carbonate Provide any relevant background balanced chemical equations and techniques used and key concepts how the reactions will allow for the determination of the unknown metal Report the number of your unknown Include the data from your gravimetric analysis Include a data table for your 3 simple weight loss trials Results Include a results table that summarizes your results from both methods be sure this includes all trials of each method Report your findings of the molar mass of your metal carbonate the molar mass atomic weight of the metal and the identity of the metal Attach hand written sample calculations to the back of your report Discussion Discuss the experiment and any possible sources of error Answer the following question as part of your discussion 1 Which method do you feel was the most successful be sure to support your answer with an explanation In keeping with the Problem section above phrase this as a recommendation of a procedure for a singlemethod determination of the identity of an unknown alkali metal carbonate 47 Prelab Using Conductivity to Find an Equivalence Point Part A Answer the following questions in your lab notebook be sure to show work for any calculations 1 When the conductivity probe is placed in a solution of BaOH2 do you expect the conductivity to be high or low Why 2 Do you expect the conductivity to increase or decrease as you add HZSO4 to the solution of BaOH2 Why 3 Do you expect the conductivity in the ask to be greater or less than the original when you have added an equal number of moles ofH2SO4 to the moles of BaOH2 originally present 4 If you add excess H2SO4past the equivalence point what should happen to the conductivity of the solution in the ask 5 Write the balanced chemical reaction for the titration of strontium hydroxide with sulfuric acid Could you use conductivity to determine the equivalence point of this reaction Why or why not Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 48 Using Conductivity to Find an Equivalence Point OBJECTIVES In this experiment you will 0 Hypothesize about the conductivity of a solution of sulfuric acid and barium hydroxide at various stages during the reaction o Use a Conductivity Probe to monitor conductivity during the reaction 0 Observe the effect of ions precipitates and water on conductivity INTRODUCTION In this experiment you will monitor conductivity during the reaction between sulfuric acid H2804 and barium hydroxide BaOH2 in order to determine the equivalence point In this reaction sulfuric acid will function as a diprotic acid and barium hydroxide as a dibasic base From the volume used and known concentration of the sulfuric acid you can find the concentration of the BaOH2 solution You will also directly observed the effect of ions precipitates and water on solution conductivity The balanced chemical equation for the reaction in this experiment is Ba2aq 2 OHaq 2 Haq SO42 aq gt BaSO4s 2 H2Ol Before reacting BaOH2 and H2SO4 are almost completely dissociated into their respective ions Neither of the reaction products however is significantly dissociated Barium sulfate is a solid precipitate and water is predominantly in its neutral molecular form As 002 M HQSO4 is slowly added to BaOH2 of unknown concentration changes in the conductivity of the solution will be monitored using a Conductivity Probe When the probe is placed in a solution that contains ions and thus has the ability to conduct electricity an electrical circuit is completed across the electrodes that are located on either side of the hole near the bottom of the probe body This results in a conductivity value that can be read by the computer interface The unit of conductivity used in this experiment is microsiemens per centimeter or uScm Prior to doing the experiment it is important for you to hypothesize about the conductivity of the solution at various stages during the reaction as you were asked to do in the Pre lab Discuss the following questions with your lab partners Would you expect the conductivity reading to be high or low and increasing or decreasing in each of these situations c When the Conductivity Probe is placed in BaOH2 prior to the addition of H2SO4 c As H2SO4 is slowly added producing BaSO4 and H20 o When the moles of HQSO4 added equals the moles of BaSO4 originally present 0 As excess HQSO4 is added beyond the equivalence point When you have reached a consensus about what will happen during the experiment proceed with the procedure below 49 MATERIALS NEEDED magnetic stirrerstir plate if available Vernier Conductivity Probe 50 mL buret 250 mL beaker Measuring Volume Using a Buret 1 2 Obtain and wear goggles Obtain approximately 60 mL of 0 02 M H2SO4 solution into a 250 mL beaker Record the precise H2SO4 concentration given in your data table CAUTION HQSO4 is a strong acid and should be handled with care Obtain a 50 mL buret and rinse the buret with a few mL of the HZSO4 solution The stopcock at the bottom is open when the handle is aligned with the tip of the buret and closed when it is at a right angle across the tip Use a utility clamp to attach the buret to the ring stand as shown here Fill the buret a little above the 000 mL level of the buret Drain a small amount of H2SO4 solution so it fills the buret tip and leaves the H2SO4 at a mark that is slightly below the 000 mL level of the buret Read and record this volume asking the TA for help if you are unsure on how to do this Dispose of the waste solution from this step as directed by your instructor Put on gloves and obtain about 60 mL of the BaOH2 solution in a clean beaker Ti BaOH2 solution and transfer the solution to a clean 100 mL beaker Then add 15 mL of distilled water to the beaker CAUTION BaOH2 is toxic Handle it with care Using a 100 mL graduated cylinder measure out 250 mL of the Arrange the buret Conductivity Probe beaker containing BaOH2 and stirring bar and plate as shown in the picture above The Conductivity Probe should extend down into the BaOH2 solution to just above the stirring bar the bar should not hit the probe when it spins quickly but smoothly leave the stir bar on at this speed Set the selection switch on the amplifier box of the conductivity probe to the 02000 uScm range Connect the Conductivity Probe to the computer interface Prepare the computer for data collection by opening the file Lab 6 Eqiv pt from the Chemistry 227 folder of LoggerPro Before adding H2SO4 titrant click and monitor the displayed conductivity value in uScm Once the conductivity has stabilized click In the edit box type the current buret reading in mL Press ENTER to store the first data pair volume conductivity for this experiment You are now ready to begin the titration This process goes faster if one person manipulates and reads the buret while another person operates the computer and enters volumes a Add about 10 mL of 002 M H2SO4 to the beaker When the conductivity stabilizes again click In the edit box type the current buret reading Press ENTER You have now saved the second data pair for the experiment b Continue adding 10 mL increments of H2SO4 each time waiting for the reading to stabilize clicking the Keep button and entering the buret reading until the conductivity has dropped below 200 uScm c After the conductivity has dropped below 200 uScm add one 05 mL increment and enter the buret reading as above 50 9 10 d Next use 2 drop increments 01 mL until the minimum conductivity has been reached at the equivalence point Read and enter the volume after each 2 drop addition When you have passed the equivalence point continue using 2 drop increments until the conductivity is greater than 50 uScm again e Finally use 10 mL increments read and enter the volume at each increment until the conductivity reaches about 2000 uScm When you have finished collecting data click Dispose of the beaker contents in the waste jar as directed by your TA Print a copy of the table Print a copy of the graph Make sure each group member has the data PROCESSING THE DATA 1 From the data table and graph that you printed determine the volume of HZSO4 added at the equivalence point The graph should give you the approximate volume at this point but recall that you must subtract the beginning volume if it wasn t 000 mL The precise volume of HZSO4 added can be confirmed by examining the data table for the minimum conductivity obtained Record the volume of H2SO4 Calculate moles of HZSO4 added at the equivalence point using the molarity M of the H2SO4 and its volume in L Calculate the moles of BaOH2 at the equivalence point Use your answer in the previous step and the ratio of moles of BaOH2 and H2SO4 in the balanced chemical equation or the 22 ratio of moles of H to moles of OH From the moles and volume 25 mL of BaOH2 used calculate the concentration of BaOH2 in molarity molL EQUIVALENCE POINT DETERMINATION An Additional Method An altemate way of determining the precise equivalence point of this titration is to perform two linear regressions on the data One of these will be on the linear region of data approaching the equivalence point and the other will be the linear region of data following the equivalence point The equivalence point volume corresponds to the volume at the intersection of these two lines 1 2 Drag your mouse cursor across the linear region of d that precedes the minimum conductivity reading Click on the Linear Fit button Drag your mouse cursor across the lineegion of data that follows minimum conductivity reading Click on the Linear Fit button Choose Interpolate from the Analyze menu Then move the mouse cursor to the volume reading when both linear fits display the same conductivity reading This volume reading will correspond to the equivalence point volume for the titration Record this volume and comment on whether it matches the one you selected manually in the Discussion section of your report This lab was modified from lab 26 Using Conductivity to find an Equivalence Point from Chemistry with Computers Third Edition Vernier Inc 51 Using Conductivity to Find an Equivalence Point Lab Report Your report for this lab should include the following sections Abstract Your abstract must be written individually and should provide the concentration of BaOH2 you determined Introduction Begin with a statement of the purpose of the experiment Provide any relevant background and key concepts balanced chemical equations and an explanation of the techniques used Data Report the molarity of sulfuric acid used Include a copy of the data table from LoggerPro Results Include a copy of your titration graph from LoggerPro Report your determined concentration of BaOH2 from both analysis methods if they differ Attach hand written sample calculations to the back of your report Discussion Discuss the experiment and any possible sources of error Answer the following question as part of your discussion 1 Which of the two analysis methods do you feel was the most accurate be sure to support your answer with an explanation 52 Prelab Atomic Emission Spectra Part A Answer the following questions in your lab notebook be sure to show work for any calculations 1 What wavelengths of the electromagnetic spectrum correspond to visible light 2 Why do atoms exhibit line spectra 3 When light is emitted from the hydrogen atom is the atom moving from a higher energy state to a lower energy state or a lower energy state to a higher energy state 4 What is the equation for the energy levels of the hydrogen atom Give the units associated with the energy equation you report Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 53 Atomic Emission Spectra Experiment Goals I To View the hydrogen emission spectrum and other atomic line spectra I To contrast the line spectra with other broadband sources I To measure the wavelengths of the bright lines in the visible emission spectra of hydrogen and mercury and calculate the energy of each line I To gain an understanding of the quantized nature of the hydrogen atom Supplies I Simple transmission grating spectroscope I Hydrogen and other elemental emission lamps I White red and green light sources Definitions 1 A spectroscope is an instrument that allows you to analyze light in some way In this case your spectroscope acts like a prism splitting the light into different wavelengths 2 A continuous spectrum is one in which a rainbow of colors is seen when viewed through a spectroscope or prism 3 A line spectrum when viewed through a spectroscope appears as one or more sharp narrow lines colors 4 A band spectrum is intermediate in appearance there will be a brightest color but a range of nearby wavelengths are emitted Background The understanding of the internal structure of the atom was advanced when Niels Bohr explained the cause of the emission spectra of atoms using the concept of quantization He stated that electrons in the atom could exist at finite energy levels but not in between them Electrons within an atom can be excited to higher energy states through various means including heating the atoms or using an electric discharge like a spark After exciting the electrons the atoms emit electromagnetic radiation as the excited electrons relax into a lower energy state The energy of the light emitted is equal to the difference between the energy levels in the atom This emitted light can be passed through a prism or re ected from a diffraction grating to spatially separate it into its individual wavelength components colors generating an atomic emission spectrum a line spectrum characteristic of the particular sample of atoms As an example the emission spectrum of hydrogen consists of only four visible lines red blue green violet and deep violet although your eyes may not be sensitive to the last one since individual s visual acuity varies Each color corresponds to the transition of an electron from an excited state a higher principal energy level to a lower principal energy level possibly the 54 ground state or some other allowed energy level in between As the electron drops it emits energy in the form of a photon which may or may not be in the Visible region In this experiment you will use a spectroscope and gas discharge lamps to measure the Wavelength of each bright line in the Visible atomic emission spectra of both hydrogen and mercury You will then use these measurements to calculate the photon energy for each bright line Recall that the energy of light is related to the Wavelength and frequency of the light thrpugh Planck s constant h 6626 X1034 J sec and the speed of light in a Vacuum c 300 X 10 msec The nal results of this experiment will be the wavelength in meters and the photon energy of each bright line measured for hydrogen and mercury 55 How To Use The Spectroscope WARNING The power supply for the discharge lamps operates at 5000 Volts DO NOT TOUCH Figure 2 shows the spectroscope you will be using It is a small box with a transparent grating in one side and a narrow slit directly opposite the grating To observe a spectrum you point the slit toward a light source and look through the grating You will see an image of the spectrum along the back wall of the box just over the wavelength scale To determine the wavelength of the light you observe you will make use of the distance of the slit to the image which together with the distance from the slit to the grating will allow you to calculate the wavelength using Equation 1 Using the scale on the back wall of the spectroscope measure the distance from the slit to the image of each line this distance is denoted a in Equation 1 The distance from the slit to the grating is b roughly the length of the box and the distance between the grating lines is d in cm For the gratings used here there are 13400 grating lines per inch Use this to find d Equation 1 7 ada2 b212 Other Equations That Might Be Useful A E 2178 x 1048 J 1Ilz nal 1Ilzinitial Rydberg equation for Balmer series nmitia1 2 A E he7x Links wavelength and energy be careful of the units 1nm1x1O399m 56 Emission Light Source Specroscope V T K 15 5393 Entrance V 31i p l p l39 7 39 39 suw1u4L w ve1engeh Scale 397 X Transmission Graizing 13439 Lines per Inch 39e 2 A slmple transmission grating spectroscope O L ighiz Source 1 Slil a 39Image 1 I b p O Jr quot OW I w Cratmg Figure 3 Measudng wavelength with a transmission grating speetroscopo a a distance from slit to Image 57 Spectrum of a Single Electron Element Hydrogen Record the line color and its position for the 3 or 4 brightest lines observed using the hydrogen lamp in table 1 and calculate each wavelength using Equation 1 from your lab manual Be sure to include a sample of your work for one of the calculations Table 1 Bright Line Spectra for Elemental Hydrogen Line Color Spectral Line Position Wavelength nm cm Caculated using Eq 1 Clearly show your work here for your calculation of wavelength Spectrum of MultiElectron Elements and Other Miscellaneous Spectra Qualitatively observe other atomic spectra and make notes below about the observed differences from the hydrogen spectrum Qualitatively compare the 3 brightest lines from the emission spectra of the elemental mercury lamp to the spectra of a fluorescent lamp and an incandescent bulb and the red and green colored sources provided Observe the solar spectra if daylight is available 58 Data Analysis 1 For the first four electronic transitions in the Balmer Series calculate the change in energy of the electron AE the predicted energy of the emitted photon Ephoton and the predicted wavelength of the emitted photon photon Put the calculated values in Table 2 and be sure to clearly show an example of each calculation in the space provided Table 2 Calculated Values for the Balmer Series of Hydrogen Electronic Transition AE J Ephoton J photon nm n3 gt n2 n4 gt n2 n5 gt n2 n6 gt n2 Clearly show the following calculations for the n3 gt n2 transition the change in energy of the electron AE the predicted wavelength of the emitted photon photon 2 Based on your theoretical calculations match the electronic transitions in the Balmer Series to the spectral lines you observed and document your choices in Table 3 Then calculate the percent error between your experimentally determined and calculated wavelengths Table 3 Comparison of Experimental and Accepted Wavelengths from the Balmer Series Spectral Line Color Observed Experimental A nm from Table 1 Accepted A nm from Table 2 Electronic Transition Percent Error n3 gt n2 n4 gt n2 n5 gt n2 n6 gt n2 59 Below clearly show your percent error calculation for the n3 gt n2 transition 3 It is not possible to observe the n7 gt n2 transition in the Balmer Series Why do you think that is 4 Emission spectra are sometimes referred to as atomic fingerprints Is it possible to use them to identify elements in an unknown sample Explain your reasoning think about the Hg spectrum and that of a fluorescent bulb 5 Why do you think sodium vapor lights cast a different color yellowish than fluorescent lamps 6 Calculate the ionization energy of the hydrogen atom Think about this process as taking an electron from its ground state n 1 to a positionenergy level far far away from the nucleus n oo 60 Atomic Emission Spectra Lab Report There is not a formal lab report for this lab Complete the above pages and submit them to your TA PreLab Determining the Concentration of a Solution Beer s Law Part A Answer the following questions in your lab notebook be sure to show your work for any calculations 1 You are given a colored solution that is labeled IM You need to prepare a solution from this that is 05 M Describe your procedure in detail 2 What is the relationship between absorbance and transmittance 3 Allura Red is a commonly used red food dye Does Allura Red transmit or absorb red light 4 If 500 mL of a 05 M solution is diluted to a final volume of 1000 ml what is the concentration of the final dilute solution Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 62 Determining the Concentration of a Solution Beer s Law OBJECTIVES In this experiment you will Prepare Allura Red standard solutions Use a Colorimeter to measure the absorbance value of each standard solution Find the relationship between absorbance and concentration of a solution Use the results of this experiment to determine the concentration of Allura Red in red Gatorade INTRODUCTION The primary objective of this experiment is to determine the concentration of Allura Red in a commercially available beverage You will be using the Colorimeter shown in Figure 1 In this device light from the LED light source will pass through the solution and strike a photocell A higher concentration of the colored solution absorbs more light and transmits less than a solution of lower concentration The Colorimeter monitors the light received by the photocell and reports either an absorbance or a percent transmittance value as compared to a blank a solution containing no absorber Ll3rIn4 r Beer39s Law Standard 1 Standard 2 Standard 3 Hhsnrbance Standard 4 Enncentratinn of I Concentration mnl L Figure 1 Figure 2 You are to prepare five Allura Red solutions of known concentration standard solutions and conduct a calibration procedure Each standard solution is transferred to a small rectangular cuvette that is placed into the Colorimeter The amount of light that passes through the solution and strikes the photocell is used to compute the absorbance of each solution When a calibration graph of absorbance vs concentration is plotted for the standard solutions a linear relationship should result as shown in Figure 2 This linear relationship between absorbance and concentration for a solution is known as Beer s law The concentration of Allura Red in an unknown solution Gatorade is then determined by measuring its absorbance in the same way with the Colorimeter By locating the absorbance of the unknown solution on the vertical axis of the graph the corresponding concentration can be found on the horizontal axis follow the arrows in Figure 2 The concentration of the unknown can also be found using the slope of the Beer s law line assuming that the y intercept of the calibration line is zero 63 MATERIALS NEEDED Vernier Colorimeter one cuvette one 10 mL pipet pipet pump or pipet bulb one 50 mL volumetric ask PROCEDURE 1 2 Obtain and wear goggles Obtain about 30 mL of Allura Red stock solution in a 100 mL beaker Add about 30 mL of distilled water to another 100 mL beaker Be sure to record the concentration of the stock solution of Allura Red from the container You will prepare five solutions of Allura Red varying in concentration from approximately 6 x 106 M to 2 x 105 M You may wish to check your concentrations and calculations with your TA before making the solutions Make the solutions by pipetting the correct quantity of the Allura Red stock solution into the volumetric ask and then filling to the line with distilled water Be careful to avoid getting liquid above the fill line Thoroughly mix each solution by inverting the stoppered ask ten times Connect the Colorimeter to the computer interface Prepare the computer for data collection by opening the file Lab 11 Beer s Law from the Chemistry 227 folder of LoggerPro Set the colorimeter to a wavelength of 470 nm You are now ready to calibrate the Colorimeter Prepare a blank by filling the cuvette 34 full with distilled water To correctly use a Colorimeter cuvette remember 0 All cuvettes should be wiped clean and dry on the outside with a tissue o Handle cuvettes only by the top edge of the ribbed sides o All solutions should be free of bubbles o Always position the cuvette with its reference mark facing toward the white reference mark at the top of the cuvette slot on the Colorimeter Blank the Colorimeter Place the cuvette with the blank water in the colorimeter In Logger Pro click on Experiment then from the drop down menu select calibrate and lab pro colorimeter In the popup window check the box next to one point calibration Click Calibrate now and enter 100 in the box provided lOO T Click Keep and then Done You should now see an absorbance reading of 0000 You are now ready to collect absorbance data for the five standard solutions Click Empty the water from the cuvette Using standard solution 1 the lowest concentration sample rinse the cuvette twice with 1 mL amounts and then fill it 34 full Wipe the outside with a tissue and place it in the Colorimeter After closing the lid wait for the absorbance value displayed on the computer monitor to stabilize Then click and type the concentration of the standard solution into the edit box and press the ENTER key The data pair you just collected should now be plotted on the graph NOTE When entering values 2 x 106 can be entered in standard computer format as 2E 6 You may need to click on the Autoscale button to rescale the graph as you go along 64 10 ll 12 13 Discard the cuvette contents in the waste jar as directed by your TA Rinse the cuvette twice with standard solution 2 next highest concentration and fill the cuvette 34 full Wipe the outside place it in the Colorimeter and close the lid When the absorbance value stabilizes click type the concentration of the standard solution in the edit box and press the ENTER key Repeat the Step 8 procedure to save and plot the absorbance and concentration values of standard solutions 35 Wait until Step 12 to do the unknown When you have entered all of your standard solutions click Be sure to record the absorbance and concentration data pairs that are displayed in the table Examine the graph of absorbance vs concentration To see if the curve represents a linear relationship between these two variables click the Linear Fit button A bestfit linear regression line will be shown for your five data points and your blank This line should pass near or through the data points and the origin 00 of the graph Note Another option is to choose Curve Fit from the Analyze menu and then select Proportional The Proportional fit has a yintercept value equal to 0 therefore this regression line will always pass through the origin of the graph Obtain a small amount of Gatorade in a small clean beaker Use the pipette to deliver 10 mL of the Gatorade to a clean dry volumetric ask Finish preparing your unknown by diluting the Gatorade to a total volume of 100 mL with distilled water and mix thoroughly Rinse the cuvette twice with the unknown solution and fill it about 34 full Wipe the outside of the cuvette place it into the Colorimeter and close the lid Read the absorbance value displayed in the meter Important The reading in the meter is live so it is not necessary to click to read the absorbance value When the displayed absorbance value stabilizes record its value Discard the solutions in the waste jar as directed by your teacher PROCESSING THE DATA You may use Microsoft Excel to plot the data and obtain a linear relationship between the data or you may use the following method 1 Determine the unknown concentration With the linear regression curve still displayed on your graph choose Interpolate from the Analyze menu A vertical cursor now appears on the graph The cursor s concentration and absorbance coordinates are displayed in the oating box Move the cursor along the regression line until the absorbance value is approximately the same as the absorbance value you recorded in Step 12 The corresponding concentration value is the concentration of the unknown solution in molL Print a graph of absorbance vs concentration with a regression line and interpolated unknown concentration displayed To keep the interpolated concentration value displayed move the cursor straight up the vertical cursor line until the tool bar is reached Enter your names and the number of copies of the graph you want and print Use the calibration curve equation to determine the concentration of Allura Red in the diluted solution solve for Concentration given Absorbance Calculate the concentration of Allura Red in the undiluted Gatorade 65 Allura Red has the following chemical structure DH D r CI HEquotHquot 11 Li M1 3 H n OE With the help of your TA calculate the molar mass of Allura Red Use the molar mass to determine what mass of Allura Red you would consume if you drank one 20 ounce bottle of Gatorade Finally determine the number of molecules of Allura Red you would consume if you drank one 20 ounce bottle of Gatorade is molar mass necessary for this step This lab was modified from lab 11 Determining the Concentration of a Solution Beer s Law from Chemistry with Computers Third Edition Vernier Inc 66 Determining the Concentration of a Solution Beer s Law Lab Report Your report for this lab should include the following sections Abstract Your abstract must be written individually and should include the concentration of Allura Red in Gatorade Introduction Begin with a statement of the purpose of the experiment Provide any relevant background and key concepts and an explanation of the techniques used Data Include a table showing the concentrations of your standard solutions and their absorbance values Results Include a copy of your calibration curve State the concentration of Allura Red in your diluted Gatorade solution and the undiluted Gatorade Attach hand written sample calculations to the back of your report Discussion Discuss the experiment and any possible sources of error Answer the following questions as part of your discussion 1 How many molecules of Allura Red would you consume if you drank one 20 ounce bottle of Gatorade 2 What mass of Allura Red you would consume if you drank one 20 ounce bottle of Gatorade 67 Electron Density Prelab Before starting this lab activity complete the following questions Your response to these questions must be attached to the activity questions when tumed in to your TA next week There is no actual chemical work and no formal lab report for this activity 1 2 Read sections 96 and 105 in your text book What are the three types of bonding What does the description polar bond refer to The interactions between water molecules can be described as electrostatic or coulombic where areas of positive charge are attracted to areas of negative charge Draw a cartoon of how three water molecules might be arranged in space given this electrostatic interaction 68 Electron Density Lab The Electron Density Lab can be accessed in two ways It is on the desktop on all of the lab computers Just click to open it and follow the instructions You can also access the lab and its images by going to the web site webpdXedushusteg This is Dr Shusterman s website for her course Scroll down the main page and you will see a link for CH 227 students This file is in pdf format You need Acrobat Reader to View it This is a free Viewer and there is a link on the page to the site where you can download it All campus computers should already have this software and most of you probably use it to View lectures and other documents If you have not already done so you might also find it useful to work through the Electron Density Tutorial also at Dr Shusterman s website This gives practice in the interpretation of electron density models Report your answers here turn in to TA after Thanksgiving Break Predict Polarity A CH4 B NH3 C CH3CH2OH D CH3NH2 E CHCICHCI F CCl2CH2 Which is most soluble in water How could you separate these compounds 69 CH228 LABS PreLab Enthalpy of Neutralization of Phosphoric Acid Part A Answer the following questions in your lab notebook be sure to show work for any calculations 1 A neutralization reaction was carried out in a calorimeter The temperature of the solution rose from 200 C to 256 C Is this reaction endothermic or exothermic A neutralization reaction was carried out in a calorimeter The change in temperature AT of the solution was 56 C and the mass of the solution was 1000 g Calculate the amount of heat energy gained by the solution qsol Use 418 JgOC as the speci c heat C5 of the solution What is the value of qreac on for the neutralization reaction described in number 2 How many moles of phosphoric acid are contained in 500 mL of 060 M H3PO4 What is the value of AHreactiOn in kJmol phosphoric acid if 500 mL of 060 M H3PO4 was used in the reaction described in number 2 Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above Work from your lab notebook and turn them into your TA 70 The Enthalpy of Neutralization of Phosphoric Acid OBJECTIVES In this experiment you will Measure the temperature change of the reaction between solutions of sodium hydroxide and phosphoric acid Calculate the enthalpy AH of neutralization of phosphoric acid Compare your calculated enthalpy of neutralization with the accepted Value Calculate the enthalpy AH of neutralization per ionizable hydrogen for phosphoric acid INTRODUCTION A great deal can be leamed by conducting an acidbase reaction as a titration In addition acidbase reactions can be observed and measured thermodynamically In this case the reaction is carried out in a calorimeter If the temperature of the reaction is measured precisely the enthalpy of neutralization of an acid by a base or Vice Versa can be determined In this experiment you will react phosphoric acid with sodium hydroxide You Will use a Styrofoam cup nested in a beaker as a calorimeter as shown in Figure 1 For purposes of this experiment you may assume that the heat loss to the calorimeter and the surrounding air is negligible Phosphoric acid will be the limiting reactant in this experiment and you will accordingly be determining the enthalpy AH of neutralization of the acid Selecting a limiting reactant helps ensure that the temperature measurements and subsequent calculations are as precise as possible Pages 246248 and 257258 in your text Will provide background information Figure 1 MATERIALS NEEDED Vernier computer interface Temperature Probe 71 PROCEDURE 1 Obtain and Wear goggles It is best to conduct this experiment in a WellVentilated room 2 Connect a Temperature Probe to Channel 1 of the Vernier computer interface 3 Start the Logger Pro program on your computer Open the le Lab 1 Phosphoric from the Chemistry 228 folder 4 Nest a Styrofoam cup in a 250 mL beaker as shown in Figure 1 Measure out 500 mL of 060 M H3PO4 solution into the foam cup CAUTION Handle the phosphoric acid with care It can cause painful burns if it comes in contact with the skin 5 Use a utility clamp to suspend the Temperature Probe from a ring stand see Figure 1 Lower the Temperature Probe into the phosphoric acid solution 6 Measure out 500 mL of 185 M NaOH solution in a graduated cylinder and transfer it to a 250 mL beaker CAUTION Sodium hydroxide solution is caustic Avoid spilling it on your skin or clothing 7 Conduct the experiment a Click to begin the data collection and obtain the initial temperature of the H3PO4 solution b After you have recorded three or four readings at the same temperature add the 500 mL of NaOH solution to the Styrofoam cup all at once Use a glass stirring rod to stir the reaction mixture gently and thoroughly c Data will be collected for 10 minutes You may terminate the trial early by clicking if the temperature readings are no longer changing d Click the Statistics button The minimum and maximum temperatures are listed in the statistics box on the graph If the minimum temperature is not a suitable initial temperature examine the graph and determine the initial temperature e Record the initial and maximum temperatures for Trial 1 f Close the Statistics box by clicking the X in the corner of the box 8 Rinse and dry the Temperature Probe Styrofoam cup and stirring rod Dispose of the solution as directed 9 Repeat Steps 48 to conduct a second trial If directed conduct a third trial Print a copy of the graph of the second trial to include with your data and analysis 72 DATA TABLE Trial 1 Trial 2 Trial 3 Maximum temperature C Initial temperature C Temperature change A7 DATA ANALYSIS 6 7 10 Write the balanced equation for the reaction of phosphoric acid and sodium hydroxide Use the equation below to calculate the amount of heat energy gained by the solution qsol In determining the mass m of the solution use 111 gmL for the density be sure to use the total Volume of the solution after the acid and base are mixed The change in temperature AT is a directional change where AT Tf Ti Use 418 JgOC as the specific heat Cs of the solution qs01Csgtltmgtlt AT The heat calculated above represents the heat gained by the solution the solution being predominantly water Since we are interested in the heat of neutralization of phosphoric acid we need the heat transfer associated with the reaction qrm If the solution gained heat the reaction must have given off heat This relationship can be expressed by the following equation qsol qrxn Determine the number of moles of phosphoric acid used in the reaction Use the moles of phosphoric acid along with qrxn to determine the enthalpy change AH for the reaction in terms of kJmol of phosphoric acid This is your experimental Value of AH AH qrmmoles H3PO4 The accepted Value for the AH of neutralization for phosphoric acid is 15644 kJmol Calculate the percent error in your experimental Value 73 The Enthalpy of Neutralization of Phosphoric Acid Lab Report Your report for this lab should include the following sections Abstract Your abstract should be Written individually Introduction Include Why you did this experiment relevant background and general equations Data Include your data table Results Report your calculated value of AH of neutralization for phosphoric acid Include a results table with results from each trial and an average value for the AH of neutralization for phosphoric acid Report the percent error for the AH of neutralization for phosphoric acid Be sure to attach hand written sample calculations to the back of your report Discussion Discuss the experiment and any possible sources of error 74 PreLab Hess s Law Part A Answer the following questions in your lab notebook be sure to show work for any calculations 1 What is the formula that relates the temperature change observed in a substance with the energy released or absorbed 2 When you measure a temperature rise during a chemical reaction is the reaction endothermic or exothermic 3 The enthalpy of the reaction for the reaction of calcium oxide with hydrochloric acid is exothermic Will the reverse reaction have a positive or negative AH 4 Hess s Law allows us to combine reactions to determine the heat of reaction for a net reaction that has not been measured For the reactions described in the lab the second reaction is difficult to measure as written You will measure the heat of reaction for the reverse reaction How will you use the measurement in the Hess s Law calculation 5 In Part A of this lab you are reacting approxamently 05 g of Mg In Part B you are directed to use a molar equivalent of MgO for the reaction What is the molar equivalent of MgO when comparing to 05 g Mg Can you think of any reasons why this might be important Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 75 Enthalpy of Reaction and Hess 39s Law Introduction In this experiment you will be finding the enthalpy of formation for MgOS using an indirect method Remember according to Hess39s Law see your textbook for more details if two or more reactions can be added to give a net reaction AH for the net reaction is simply the sum ofthe AH quots for the reactions which are added Consider the following three reactions 1 MgS 2Haq gt Mgz aq HM AHO1 2 Mg2aq H200 gt MgOS 2Haq AHO2 3 Hg 120 gt Hzgm AH03 4 MgS l202 gt MgOS AHO4 You will determine the heat of reaction for reactions 1 and 2 experimentally then use the known Value ofthe enthalpy of formation of water AHO3 2859 kl to calculate AHO4 which is the enthalpy of formation of MgO Be aw are that equation 2 is the reverse of the reaction you actually run and measure Note the enthalpy of formation of MgO cannot easily be measured Experimental Procedure Part A 1 Obtain a coffee cup calorimeter from the stockroom Make sure the cup is clean and dry Nest a Styrofoam cup in a 250 mL beaker and put 50 mL of 10 M HCl into the calorimeter 2 Using a weighing boat weigh out a sample containing between 045 055 grams of magnesium 3 Connect a Temperature Probe to Channel 1 of the Vernier computer interface Start the Logger Pro program on your computer Open the file Lab 1 Phosphoric from the Chemistry 228 folder 4 Use a utility clamp to suspend the Temperature Probe from a ring stand see Figure 1 Lower the Temperature Probe into the acid solution 5 Conduct the experiment 76 9 e f Click to begin the data collection and obtain the initial temperature of the acid solution After you have recorded three or four readings at the same temperature add the magnesium to the styrofoam cup all at once Use a glass stirring rod to stir the reaction mixture gently and thoroughly Data will be collected for 10 minutes You may terminate the trial early by clicking if the temperature readings are no longer changing Click the Statistics button The minimum and maximum temperatures are listed in the statistics box on the graph If the minimum temperature is not a suitable initial temperature examine the graph and determine the initial temperature Record the initial and maximum temperatures for Trial 1 Close the Statistics box by clicking the X in the corner of the box 6 Rinse and dry the Temperature Probe Styrofoam cup and stirring rod Dispose of the solution as directed 7 Conduct another trial as above Make sure the calorimeter is clean and mostly dry before repeating the experiment Caution Wear your goggles at all times HCl is a strong acid Hydrogen gas is ammable Do not use any open ames in the lab Part B Repeat the above procedure this time replacing Mg with MgO Use a clean dry calorimeter You should use a molar equivalent of MgO 243 g Mg is the molar equivalent of 403 g MgO why your measurement should be within 5 Be certain all the MgO dissolves this will require vigorous stirring Conduct another trial as above Calculations To relate heats of reactions in energy units of Joules with temperature differences we USE qmxSxAT For the reactions above it is a good approximation to take specific heat of the solution to be the specific heat of water S 4184 Ig C For mass because you are using the specific heat of pure water use the mass of the water only not the combined mass of water and solute Calculate q for reaction 1 and 2 Report AH rxn for reaction 1 and 2 be certain to use units of kmol Calculate AH 4 77 Hess s Law Lab Report Your report for this lab should include the following sections Abstract Your abstract should be written individually Introduction Include why you did this experiment relevant background and general equations Data Prepare a data table that includes the initial and final temperatures for each trial Report the mass of Mg and MgO used in each trial Results Prepare a results table showing the calculated AH for each trial and averages for reactions one and two and the value of AH for reaction four Be sure to attach hand written sample calculations to the back of your report Discussion Discuss the experiment and any possible sources of error As part of your discussion answer the following questions 1 For an exothermic reaction does the temperature observed rise or fall 2 For an exothermic reaction is AH0 positive or negative 3 Is reaction 1 endothermic or exothermic reaction 2 4 In this lab you measure the quantity q How is this different from AHOIXH remember the definitions and units Answer the following question and attach it to your report Further Analysis An alloy a metal mixture containing magnesium and another metal that does not react with hydrochloric acid needs to analyzed You are asked to determine the percent magnesium in the alloy a Describe the procedure you would use to determine the percent magnesium in the alloy and b if the sample were 30 magnesium calculate the heat evolved ifa 5 gram sample were analyzed in that manner 78 Deriving the Gas Laws Using Computer Simulations Introduction According to the kinetic molecular theory gases are in constant and random motion with enough kinetic energy such that they rarely interact with one another When gas particles collide with the walls of a container they rebound with no apparent loss of energy These characteristics describe an quotIdeal Gasquot Experimental evidence suggests that many common gases making up air behave in this manner when studied at temperatures well above their boiling points We are constantly being exposed to the behavior of gases Each time we pump up a tire blow up a balloon use a spray can or experience the cooling of gases as they escape from a gas storage container we are reminded of how gases behave with changes in temperature T volume V pressure P or number of particles n The behavior of gases has been scientifically investigated starting with Robert Boyle39s work in 1662 followed by Jacques Charles 1787 and Joseph GayLussac39s work 1802 Together these studies led to the so called quotGas Lawsquot which relate volume V pressure P temperature T and numbers of particles of gas n In a scientific manner one can derive the mathematical relationships that exist between these variables by holding two of the variables constant changing one and monitoring the effect on the fourth variable To derive the relationships you will be using an interactive researchbased simulation produced by the PhET project at the University of Colorado PROCEDURE 1 Pressure Volume Relationship 1 Go to the Physics Education Technology from the University of Colorado at httpphetcoloradoedunewsimulationssimsphpsimGas Properties 2 Click the RUN NOW button under the Gas Properties Simulation window highlighted in green 3 Play around with the simulator and see what sorts of tools are available to you to analyze the behaviors of gases Qualitatively get a feel for the relationships that exist between the four variables that describe gases P V n and T If you ever get to a point that you need to reset the simulator you can always hit the reset button at the bottom right of the screen 4 If you have not already done so on the lower right side of the screen click on the RESET button 5 On the right side of the screen click on the MEASURMENT TOOLS button Next click on the RULER option to activate the ruler 79 Gas Properties 308 ile uelp Constant Parameter CI Volume 0 Pressure C Temperature G3 None Gas in Chamber Heaw Sneciesl I Light Species IJ Gravllv I Luls Tools 8 Options MNanced Options 3 Gas in Pump 33 Hhmy Species C7 Lllgnt species 39 39 F39hET F39h3939i5 Elu i 1 I3a39 L51393939LE1lI39ill39IF39l39IE Gas F39nn3r39lies j39EIjIE39j 39jI 0 11IIIE Iquot391 iJava Application Window 6 In the upper right hand comer click on the TEMPERATURE button under the Constant Parameter heading This will hold temperature constant while allowing you to observe the relationship between pressure and Volume Gas Properties 308 Eile elp Constant Parameter C Volume 0 Pressure C3 Temperature C None Gas in Chamber Heavy Species I N Light Species I I Lots Tools 8 Options ltlt Hide Tools I Layer tool Ruler I Species information I Stopwatch I ImI gy histograms I Center of mass markers Heat Control Gas in Pump quot3 3 Light Species Remove gtJ ava Application Window 39 F39hET F39l39r39inEnluu 39E J I3asLaI39ILaln39IithF39hE 13E1SF39r39IIn3r39iIlIE1SIElElI 399 quot 11II4 Ar1 7 Using the mouse and the right button drag the ruler into a position that will allow you to measure the length of the container 8 Using the mouse and the right button grab hold of the man pushing against the container and expand the length of the container so that it measures 90 cm Record this as your initial length the height of the box will remain 50 cm and the width of the box will remain 5 0 cm 80 Gas Properties 303 Eile Help Constant Parameter CI Volume C Pressure 39 Temperature C None Gas in Chamber Heavy Species 11 E Light Species 11 Gravity I Lots Tools 8 Options lt Hide Tools GRAB AND D Layer tool Ruler Species information D Stopwatch I Energy histograms DRAG Gas in puma I Center of mass markers E quotDEW spgcies Advanced Options gtgt 73 Light Species Java Application Window A i Star I 39 39 F39hET F39h3939i5 Eulu I E Eas LaI39 Lat 39Iith F39hE Gas F39r39n3r39tiv3 EiEiI 393 Py 1111 Ar1 9 Using the mouse and the right button grab hold of the pump handle and inject one cycles Worth of gas into the chamber by pulling the handle up then pushing it back down MOVE UP THEN DOWN Gas Properties 308 Constant Parameter C ufolume C Pressure 39339 Temperature C None Gas in Chamber Heavy Species 81 E Light Species I Gravity I Lots Tools 8 Options lt Hide Tools I Layer tool Ruler II Species information II Stopwatch II Energy histograms I Center of mass markers Gas in Pump 55 H 393a7quotV 5393quotquot5ii 3539 Advanced Options gtgt 393 Light Species W Reset C I F39i39IET F39i39I39395iI39 EIiIJn 1 E I23E15 L51393939 Lat I39iti39I F39i39IE 33 F39r39ini39liE lEiEil I24quot P Java Application Window 11n4 Ar1 81 10 Once the pressure has somewhat stabilized record your pressure Value for the chamber length of 90 cm This will represent your initial pressure in atmospheres Gas Properties 303 ile elp 1 39393939 g iFrisJ Constant Parameter CD Volume C Pressure Temperature 0 None Gas in Chamber Heavy Species 31 Light Species I I Lots Tools 8 Options ltlt Hide Tools I Layer tool Ruler I Species information II Stopwatch PRESSURE ATM I Energy histograms I Center of mass markers Gas in Pump 3 H 9a7 W39599ci 5 ndiranced Options gtgt Cquot Liglit Species gtJava Application Window 39 39 F39hET F39i39i3939ii Eiiui 1 E 355 L339I Lat Iiith F39hE 3a F39riiperties lEiEil 11 Using the mouse and right button grab hold of the man pushing on the container and decrease the length of the container to approximately 80 cm Once the pressure has stabilized again this may take a short period of time to happen record the new pressure for a length of 80 cm Gas Properties 308 ile elp Constant Parameter 0 Volume O Pressure Ci Temperature C None Gas in Chamber 7 Heaiiy Species 31 H PrESSurE Light Species UN I quot Gravity gm V 39 I Lots Tools 3 Options ltlt Hide Tools D Layer tool Ruler PUSH IN E Species information D Stopwatch B Energy histograms Gas in punm E Center of mass markers E H3quot3a quot3939939 Species advanced Options gtgt Liglit Species Java Application Window L 39 39 F39hET F39l39i3939ii Eiui 1 E 335 Lal39I Lat Iu39uith F39hE i3a F39riper39ties iEIIIEi 82 12 Repeat step 9 for approximate lengths of 70 cm 60 cm 50 cm 40 cm 30 cm and 20 cm you will probably not get to exactly 2 cm For each trial record the length Value and resulting pressure value in a properly labeled data table 13 Record any qualitative observations on the behavior of the gas molecules as the volume decreases 14 Click the RESET button to remove all the gas particles from the chamber before moving on to the next section PROCEDURE 2 Volume Temperature Relationship Devise an experiment using the simulator in which you can elucidate the relationship between Temperature and the Volume of a gas Collect and record your data over a wide range of temperatures 0600 K in a properly labeled table Procedure 3 Temperature Pressure Relationship Devise an experiment using the simulator in which you can elucidate the relationship between Temperature and the Pressure of a gas Collect and record your data over a wide range of temperatures 0600 K in a properly labeled table Procedure 4 Pressure Quantity Relationship Devise an experiment using the simulator in which you can elucidate the relationship between Quantity and the Pressure of a gas Collect and record your data over a wide range of number of molecules in a properly labeled table Analysis For this lab you will need to submit neat labeled data tables for each procedure You must also submit a graphical representation for each relationship Be sure to label each axis and include a title for each graph Please see the information on pages 15 and 16 of this lab manual I suggest that you utilize Microsoft Excel or some other comparable spreadsheet software to produce your tables and graphs Along with the graphs and tables for each procedure answer completely the questions below that correlate with each section Analysis Procedure 1 Pressure Volume Relationship 1 Graphically represent the Pressure atm Volume cm3 relationship with volume on the xax1s 2 Graphically represent the Pressure atm and Inverse Volume 1V cm 3 relationship with IN on the x axis 3 Identify the mathematical relationship that exists between pressure and volume when temperature and quantity are held constant as being directly proportional or inversely proportional Explain your answer and write an equation that relates pressure and volume to a constant 83 5 Why were you asked to graph pressure and the inverse of volume Calculate the slope of the line for your pressure vs 1volume graph What does this number represent Would you expect it to be the same for all gases Explain your answer 539 Analysis Questions Procedure 2 Volume Temperature Relationship 6 Graphically represent the Temperature K Volume cm3 relationship 7 Identify the mathematical relationship that exists between volume and temperature when pressure and quantity are held constant as being directly proportional or inversely proportional Explain your answer and write an equation that relates volume and temperature to a constant 8 Calculate the slope of the line for your temperature vs volume graph What does this number represent Would you expect it to be the same for all gases Explain your answer Analysis Questions Procedure 3 Temperature Pressure Relationship 9 Graphically represent the Temperature K Pressure atm relationship Make sure the axis that represents temperature includes a range from O K to 600 K 10 Identify the mathematical relationship that exists between pressure and temperature when volume and quantity are held constant as being directly proportional or inversely proportional Explain your answer and write an equation that relates pressure and temperature to a constant 11 Calculate the slope of the line for your temperature vs pressure graph What does this number represent Would you expect it to be the same for all gases Explain your answer 12 Explain the effects of temperature on molecular motion Using this explanation explain why both pressure and volume can decrease with decreasing temperature 13 Absolute zero is theorized to be the temperature that all molecular motion stops Based on this what would you predict to be the pressure and volume of a gas sample whose temperature is decreased to absolute zero Explain Analysis Questions Procedure 4 Pressure Quantity Relationship 14 Graphically represent the Quantity number of molecules Pressure atm relationship 15 Describe the impact of increasing the number of molecules or moles of a gas on the pressure of a gas sample Would you expect this trend to be the same for all gases Explain your answer 16 Based on your previous observations predict the impact of changing the number of moles of a gas sample on the volume of the gas sample if pressure and temperature are held constant and on the temperature of a gas sample if pressure and volume are held constant Explain your answer 84 PreLab Decomposition of Hydrogen Peroxide Part A Answer the following questions in your lab notebook be sure to show work for any calculations 1 2 What is Dalton s law of partial pressure A mixture of three gasses A B and C has a total pressure of 849 torr and the partial pressure of A is 57 torr and the partial pressure of B is 573 torr What is the partial pressure of C A gas has a volume of 94 mL a pressure of 743 torr and a temperature of 20 C Calculate the number of moles of gas present Assume ideal behavior of the gas If 000946 moles of 02 gas is collected from the decomposition of hydrogen peroxide how many moles of hydrogen peroxide were reacted Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 85 Decomposition of Hydrogen Peroxide OBJECTIVES Decompose hydrogen peroxide using KI as a catalyst Measure the Volume of oxygen gas generated through the decomposition reaction Illustrate Dalton s Law of partial pressure Determine the number of moles of oxygen gas produced using the ideal gas law Determine the percent hydrogen peroxide in an aqueous solution INTRODUCTION Hydrogen peroxide spontaneously decomposes to form oxygen gas according to the following equation 2 H202 aq gt 2 H20 1 02 g This process usually occurs Very slowly Many different compounds or ions are capable of acting as catalysts increasing the rate of the reaction Here potassium iodide KI will be used as a catalyst to make the reaction produce products rapidly enough to study the reaction in the lab The apparatus we will use to collect oxygen gas in this experiment is shown in figure 1 Hydrogen peroxide will be placed in the Erlenmyer ask The catalyst K1 is located in the syringe and can be added to the Erlenmyer ask to initiate the reaction As the reaction proceeds oxygen gas will be produced in the Erlenmyer ask and travel through the tubing The gas will be collected in the graduated cylinder The graduated cylinder is initially filled with water As the gas enters the cylinder it displaces water allowing the Volume of the gas to be measured Figure 1 86 MATERIALS NEEDED 125 mL Erlenmyer ask 100 mL graduated cylinder Rubber stopper with adaptors tubing and syringe PROCEEDURE 1 Place an 125 mL Erlenmyer ask on a balance and tare the scale Add approximately 5 g of hydrogen peroxide solution into the Erlenmyer ask Record the actual mass used Obtain a ring stand and clamp the ask as shown in figure 1 Place the rubber stopper tightly in the ask this should be air tight 2 Place approximately 400 mL of water in an 800 mL beaker 3 Completely fill a 100 mL graduated cylinder with water Cover the cylinder with parafilm and invert the cylinder in the 800 mL beaker Carefully clamp the cylinder in place such that the opening of the cylinder is below the surface of the water in the beaker Remove the parafilm and carefully place the end of the tubing just inside the graduated cylinder as shown in figure 1 The graduated cylinder should be completely filled with water If there is a small amount of air present in the cylinder record the volume If there is more than 10 mL of air in the cylinder you will need to redo the setup 4 In a small beaker obtain a small amount approximately 10 mL of 05 M KI Draw up 3 mL of the KI solution into the syringe Attach the syringe to the adaptor in the rubber stopper 5 Initiate the reaction by depressing the stopper on the syringe and adding the KI to the hydrogen peroxide 6 Allow the reaction to proceed until no further production of oxygen gas is observed around 10 to 15 minutes 7 Measure and record the temperature of the water 8 Record the final level of the water in the graduated cylinder Be sure to record your measurement to 2 decimal places 9 Repeat the above procedure two more times for a total of three trials At least two of your trials should agree well with one another DATA ANALYSIS 1 Determine the pressure of the oxygen gas Because the oxygen gas was collected over water some of the gas collected is water vapor The gas collected is therefore a mixture of both oxygen and water The total pressure of the gas is the sum of the pressures exerted by the oxygen gas and water vapor To determine the pressure of oxygen gas we must apply Dalton s law of partial pressure PTot P02 PH2O In other words you can find the pressure of oxygen gas by subtracting the partial pressure of water at the temperature of the water also known as the vapor pressure of water from the total pressure or atmospheric pressure Your TA will provide the 87 current barometric pressure A table of the Vapor pressure of Water at Various temperatures follows Table 1 Vapor pressure of Water at Various temperatures Vapor Vapor Vapor Temperature Pressure Temperature Pressure Temperature Pressure C torr C torr C torr 15 128 21 186 27 267 16 136 22 198 28 283 17 145 23 21 1 29 30 18 155 24 224 30 318 19 165 25 238 31 337 20 175 26 252 32 357 2 Determine the Volume of the oxygen gas When the reaction was initiated 3 mL of K1 solution was added This Volume needs to be subtracted from the Volume of gas collected If your initial Volume of gas was not zero this must also be taken into consideration V02 V nal Vinitia1 3 IIIL 3 Calculate the number of moles of oxygen gas generated Now that the pressure Volume and temperature of the gas are known the moles of gas can be calculated using the ideal gas law The temperature of the gas will be considered to be the same temperature as the Water temperature measured during the experiment PV nRT 4 Calculate the amount of H202 Using the balanced equation calculate the number of moles of H202 present in the initial solution Calculate the molar mass of H202 and determine the grams of H202 present in the initial solution 5 Calculate the mass percent of hydrogen peroxide Using the mass of H202 calculated above and the initial mass of the H202 solution calculate the mass percent H202 in the initial solution 88 Hydrogen Peroxide Lab Report Your report for this lab should include the following sections Abstract Your abstract should be Written individually Introduction Include Why you did this experiment relevant background and general equations Data Include a data table with data from all 3 trials Results Include a results table with the mass percent of hydrogen peroxide from each trial Be sure to attach hand written sample calculations to the back of your report Discussion Discuss the experiment and any possible sources of error 89 PreLab Vapor Pressure and Heat of Vaporization Part A Answer the following questions in your lab notebook be sure to show work for any calculations 1 When using the equation P1T1 P2T2 to relate temperature and pressure of a gas what must be held constant 2 A sample of gas is held in a capped ask At 25 C the pressure is 693 mmHg What is the pressure of the gas at 37 C 3 If the heat of vaporization of water is 407 kJmol how much energy is required to vaporize 50 g of liquid water at 100 C 4 Would you expect most of the components in a perfume to have a low or high vapor pressure Explain Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 90 Vapor Pressure and Heat of Vaporization When a volatile liquid is placed in a container and the container is sealed tightly a portion of the liquid will evaporate The newly formed gas molecules exert pressure in the container while some of the gas condenses back into the liquid state If the temperature inside the container is held constant then at some point a physical equilibrium will be reached At this equilibrium the rate of condensation is equal to the rate of evaporation The pressure at equilibrium is called vapor pressure and will remain constant as long as the temperature in the container does not change In mathematical terms the relationship between the vapor pressure of a liquid and temperature is described in the Clausius Clayperon equation AHVa 1 lnP P C R T where ln P is the natural logarithm of the vapor pressure AHVap is the heat of vaporization R is the universal gas constant 831 JmolK T is the temperature in Kelvin and C is a constant not related to heat capacity Thus the Clausius Clayperon equation not only describes how vapor pressure is affected by temperature but it relates these factors to the heat of vaporization of a liquid AHVap is the amount of energy required to cause the vaporization of one mole of liquid at constant pressure In this experiment you will introduce a specific volume of a volatile liquid into a closed vessel and measure the pressure in the vessel at several different temperatures By analyzing your measurements you will be able to calculate the AHVap of the liquid OBJECTIVES In this experiment you will Measure the pressure inside a sealed vessel containing a volatile liquid over a range of temperatures Determine the relationship between pressure and temperature of the volatile liquid Calculate the heat of vaporization of the liquid Figure 1 91 MATERIALS Vernier Gas Pressure Sensor Temperature Probe rubber stopper assembly plastic tubing with two connectors PROCEDURE 1 10 ll Obtain and wear goggles CAUTION The alcohol used in this experiment is ammable and poisonous Avoid inhaling the vapors Avoid contact with your skin or clothing Be sure that there are no open ames in the room during the experiment Notify your teacher immediately if an accident occurs Use a hot plate to heat 20O mL of water in a 400 mL beaker Prepare a room temperature water bath in an 800 mL beaker The bath should be deep enough to completely cover the gas level in the 125 mL Erlenmeyer ask Connect a Gas Pressure Sensor to Channel 1 of the Vernier computer interface Connect a Temperature Probe to Channel 2 of the interface Start the Logger Pro program on your computer Open the file Lab 4 Vapor Pressure from the Chemistry 228 folder Use the clear tubing to connect the white rubber stopper to the Gas Pressure Sensor About onehalf tum of the fittings will secure the tubing tightly Twist the white stopper snugly into the neck of the Erlenmeyer ask to avoid losing any of the gas that will be produced as the liquid evaporates see Figure 1 Important Open the valve on the white stopper Your first measurement will be of the pressure of the air in the ask and the room temperature Place the Temperature Probe near the ask When the pressure and temperature readings stabilize record these values Condition the Erlenmeyer ask and the sensors to the water bath a Place the Temperature Probe in the room temperature water bath b Place the Erlenmeyer ask in the water bath Hold the ask down into the water bath to the bottom of the white stopper c After 30 seconds close the valve on the white stopper Obtain a small amount of ethanol Draw 3 mL of ethanol into the 20 mL syringe that is part of the Gas Pressure Sensor accessories Thread the syringe onto the valve on the white stopper see Figure 1 Add ethanol to the ask a Open the valve below the syringe containing the 3 mL of ethanol b Push down on the plunger of the syringe to inject the ethanol c Quickly pull the plunger back to the 3mL mark Close the valve below the syringe d Carefully remove the syringe from the stopper so that the stopper is not moved Gently rotate the ask in the water bath for a few seconds using a motion similar to slowly stirring a cup of coffee or tea to accelerate the evaporation of the ethanol 92 12 13 14 15 16 17 Monitor and collect temperature and pressure data Click I callat to begin data collection Hold the ask steady once again Monitor the pressure and temperature readings When the readings stabilize click Record these values in your notebook PP99 Add a small amount of hot Water from the beaker on the hot plate to Warm the Water bath by 3 5 C Use a spoon or a dipper to transfer the hot Water Stir the Water bath slowly with the Temperature Probe Monitor the pressure and temperature readings When the readings stabilize click Record these values in your notebook Repeat Step 13 until you have completed five total trials Add enough hot Water for each trial so that the temperature of the Water bath increases by 35 C but do not Warm the Water bath beyond 40 C because the pressure increase may pop the stopper out of the ask If you must remove some of the Water in the bath do it carefully so as not to disturb the ask After you have recorded the fifth set of readings open the valve to release the pressure in the ask Remove the ask from the Water bath and take the stopper off the ask Dispose of the ethanol as directed Click to end the data collection Record the pressure readings as Ptotal and the temperature readings in your data table Do not exit the Logger Pro program until you have completed 14 of the Data Analysis section DATA TABLE Initial Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Ptotal Pair Pvap kPa Temperature C 93 DATA ANALYSIS 1 The Pair for Trials 25 must be calculated because the temperatures were increased As you warmed the ask the air in the ask exerted pressure that you must calculate Use the gas law relationship shown below to complete the calculations Remember that all gas law calculations require Kelvin temperature Use the Pair from Trial 1 as Pi and the Kelvin temperature of Trial 1 as Ti EZE T1 T2 3 Calculate and record the Pvap for each trial by subtracting Pair from Piaiai 4 Prepare and print a graph of Pvap y axis vs Celsius temperature x axis a Disconnect your Gas Pressure Sensor and Temperature Probe from the interface b Choose New from the File menu An empty graph and table will be created in Logger Pro c Double click on the x axis heading in the table enter a name and unit then enter the five values for temperature C from your data table above d Double click on the y axis heading in the table enter a name and unit then enter the five values for vapor pressure from your data table above e Does the plot follow the expected trend of the effect of temperature on vapor pressure Explain 5 In order to determine the heat of vaporization AHVap you will first need to plot the natural log of Pvap vs the reciprocal of absolute temperature Choose New Calculated Column from the Data menu Create a column ln vapor pressure Create a second column reciprocal of absolute temperature 1Temperature C 273 On the displayed graph click on the respective axes and then select ln vapor pressure to plot on the y axis and reciprocal of absolute temperature to plot on the x axis Autoscale the graph if necessary e Calculate the linear regression best f1t line equation for this graph Calculate AHVap from the slope of the linear regression f Prepare and print a second graph 99 6 The accepted value of the AHVap of ethanol is 4232 lltJmol Compare your experimentally determined value of AHVap with the accepted value 94 Vapor Pressure and Heat of Vaporization Lab Report Your report for this lab should include the following sections Abstract Your abstract should be written individually Introduction Include why you did this experiment relevant background and general equations Data Include your data table Results Include a copy of your graph of Pvap VS T OC Include a copy of your graph of ln Pvap VS 1T K Report your Value for AHVap of ethanol Report the percent error in your calculated Value of AHVap for ethanol Be sure to attach hand written sample calculations to the back of your report Discussion Discuss the experiment and any possible sources of error 95 PreLab Using FreezingPoint Depression to Find Molecular Weight Part A Answer the following questions in your lab notebook be sure to show work for any calculations 1 What is a colligative property 2 Give the equation for freezing point depression and indicate the units for each term in the expression 3 A measurement of the freezing temperature of a solution allows you to calculate the concentration of the solution What else do you need to measure to determine the molar mass of the solid added to the solvent 4 A student adds 1504 g of a solid to 250 mL of water The freezing temperature is measured to be 120 C What is the molality of the solution 5 What is the molar mass of the solid above Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 96 Using FreezingPoint Depression to Find Molecular Weight When a solute is dissolved in a solvent the freezing temperature is lowered in proportion to the number of moles of solute added This property known as freezing point depression is a colligative property that is it depends on the ratio of solute and solvent particles not on the nature of the substance itself The equation that shows this relationship is AT Kfm where AT is the freezing point depression KflS the freezing point depression constant for a particular solvent 828 C kgmol for t butanol in this experiment1 and m is the molality of the solution in mol solutekg solvent In this experiment you will first find the freezing temperature of the pure solvent t butanol C4H10O You will then add a known mass of aspirin to a known mass of t butanol and determine the lowering of the freezing temperature of the solution By measuring the freezing point depression AT and the mass of aspirin you can use the formula above to find the molar mass of the aspirin solute in gmol OBJECTIVES In this experiment you will o Determine the freezing temperature of pure t butanol Determine the freezing temperature of a solution of aspirin and t butanol Examine the freezing curves for each Calculate the experimental molar mass of aspirin Compare it to the accepted molar mass for aspirin Figure 1 MATERIALS Temperature Probe Copper stirrer 1 The ComputerBased Laboratory Journal of Chemical Education Software 1988 Vol lA No 2 p 73 97 PROCEDURE 1 2 Obtain and wear goggles Connect the Temperature Probe to the computer interface Prepare the computer for data collection by opening the file l5 Freezing Pt Depression from the Chemistry with Computers folder Part I Freezing Temperature of Pure Tbutanol 3 Add about 175 mL of tap water 250 mL beaker Using a hot plate warm the water to a temperature of about 35 C The tbutanol used in this experiment is ammable Do not use Bunsen burners during this lab Weigh a CLEAN DRY test tube It can be propped in a plastic 250 mL beaker to facilitate measuring this is useful when the tube is not empty Add 3 mL of tbutanol to your test tube using a dry Pasteur pipette Do this carefully so that you do not get any tbutanol on the upper portion of the test tube MAKE SURE IT LOOKS LIKE 3 mL compare to a sililar test tube with 3 mL of water in it Warm the tbutanol to 35 C for three minutes Insert the Temperature Probe into the hot tbutanol About 30 seconds are required for the probe to warm up to the temperature of its surroundings and give correct temperature readings During this time fasten the utility clamp to the ring stand so the test tube is above the water bath Then click to begin data collection Prepare a large beaker 400600 mL with a coldwater bath Fill beaker 23 full with cold tap water Lower the test tube into the water bath Add a small amount of ice to your water bath to bring the temperature down Make sure the water level outside the test tube is higher than the tbutanol level inside the test tube With a very slight up and down motion with the copper stirrer continuously stir the tbutanol during the cooling Continue with the experiment until data collection has stopped 10 minute run Use the hot water bath to melt the probe out of the solid tbutanol Do not attempt to pull the probe out this might damage it Carefully wipe any excess tbutanol liquid from the probe with a paper towel or tissue Weigh the test tube and t butanol To determine the freezing temperature of pure tbutanol you need to determine the mean or average temperature in the portion of graph with nearly constant temperature Move the mouse pointer to the beginning of the graph s at part Press the mouse button and hold it down as you drag across the at part of the curve selecting only the points in the plateau Click on the Statistics button The mean temperature value for the selected data is listed in the statistics box on the graph Record this value as the freezing temperature of tbutanol Close the statistics box Part II Freezing Temperature of a Solution of Aspirin and Tbutanol 10 11 Store your data by choosing Store Latest Run from the Experiment menu Hide the curve from your first run by clicking on the vertical axis label and unchecking the appropriate box Click Repeat steps 5 10 so that you have two trials of the freezing point tbutanol Measure out approximately 02 grams of aspirin into a weighing boat Using a funnel add the aspirin to the t butanol already in the 4 test tube The purpose of the funnel is to prevent aspirin from sticking to the inside wall of the test tube where it would be difficult or impossible to get into solution Be careful not to get the funnel stem into the solvent If you do you will have to dump the solvent clean and dry your apparatus and start all over by weighing out a new portion of tbutanol Determine the mass of the aspirin by weighing the 98 12 13 14 test tube solvent and aspirin It may take several minutes for the aspirin to dissolve Heat the test tube gently with hot water and agitate very gently be careful not to splash until dissolution is complete Repeat Steps 38 to determine the freezing point of this mixture When you have completed Step 8 click on the Examine button To determine the freezing point of the aspirin t butanol solution you need to determine the temperature at which the mixture initially started to freeze Unlike pure t butanol cooling a mixture of aspirin and t butanol results in a gradual linear decrease in temperature during the time period when freezing takes place As you move the mouse cursor across the graph the temperature y and time x data points are displayed in the examine box on the graph Locate the initial freezing temperature of the solution as shown here Record the freezing point in your data table Freezing Point Temperature Time Repeat the process with by adding another portion of asprin To print a graph of temperature vs time showing all data runs a Click on the vertical axis label of the graph To display both temperature runs click More and check the Run 1 and Latest Temperature boxes Click b Label both curves by choosing Text Annotation from the Insert menu and typing T butanol or Aspirin t butanol mixture in the edit box Then drag each box to a position on or near its respective curve c Print the graph PROCESSING THE DATA METHOD 1 1 Determine the difference in freezing temperatures At between the pure t butanol t1 and the mixture of t butanol and aspirin t2 Use the formula At t1 t2 Calculate molality m in molkg using the formula At Kf II1Kf 828 C kgmol for t butanol Calculate moles of aspirin solute using the answer in Step 2 in molkg and the mass in kg of t butanol solvent Calculate the experimental molar mass of aspirin in gmol Use the original mass of aspirin from your data table and the moles of aspirin you found in the previous step Compare your experimentally determined molar mass of aspirin with the known value Calculate the percent error 99 PROCESSING THE DATA METHOD 2 Here is another method that can be used to determine the freezing temperature from your data in Part 11 With a graph of the Part II data displayed use this procedure 1 Move the mouse pointer to the initial part of the cooling curve where the temperature has an initial rapid decrease before freezing occurred Press the mouse button and hold it down as you drag across the linear region of this steep temperature decrease Click on the Linear Fit button Now press the mouse button and drag over the next linear region of the curve the gently sloping section of the curve where freezing took place Press the mouse button and hold it down as you drag only this linear region of the curve Click again The graph should now have two regression lines displayed Choose Interpolate from the Analyze menu Move the mouse pointer left to the point where the two regression lines intersect When the small circles on each cursor line overlap each other at the intersection the temperatures shown in either examine box should be equal to the freezing temperature for the aspirin t butanol mixture Use the temperature to calculate AT and your molar mass for aspirin Compare your results from the two methods 100 Freezing Point Depression Lab Report Your report for this lab should include the following sections Abstract Your abstract should be Written individually Introduction Include Why you did this experiment relevant background and general equations Data Include a data table with all necessary mass measurements Include graphs for the freezing of t butanol and t butanol aspirin solution Results Report the freezing point of pure t butanol Report your calculated molar mass of aspirin Determine the percent error in your calculated molar mass the actual molar mass of aspirin is 1802 gmol Calculate the percent error in your determined molar mass of aspirin Be sure to attach hand written sample calculations to the back of your report Discussion Discuss the experiment and any possible sources of error 101 PreLab The Rate and Order of a Chemical Reaction Part A Answer the following questions in your lab notebook be sure to show work for any calculations 1 Write the general form of the rate law for the reaction you will be studying this week 2 A first order reaction has a rate constant of 290 X 104 s 1 Calculate the halflife for this reaction 3 What is the overall order of a reaction that has the following rate law Rate A2B 4 For a reaction where the general form of the rate law is rate AmB the following data were collected What is the order of the reaction with respect to A What is the order of the reaction with respect to B Initial Rate A B 001 Ms 0025 M 0025 M 001 Ms 0025 M 0050 M 009 Ms 0075 M 0025 M Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 102 The Rate and Order of a Chemical Reaction OBJECTIVES In this experiment you will Conduct the reaction of KI and FeCl3 using Various concentrations of reactants Determine the order of the reaction in KI and FeCl3 Determine the rate law expression for the reaction INTRODUCTION A basic kinetic study of a chemical reaction often involves conducting the reaction at Varying concentrations of reactants In this way you can determine the order of the reaction in each species and determine a rate law expression Once you select a reaction to examine you must decide how to follow the reaction by measuring some parameter that changes regularly as time passes such as temperature pH pressure conductance or absorbance of light In this experiment you will conduct the reaction between solutions of potassium iodide and iron III chloride The reaction equation is shown below in ionic form 2 r aq 2 Fe3 aq gt 12 aq 2 Fe2 aq As this reaction proceeds it undergoes a color change that can be precisely measured by a Colorimeter see Figure 1 By carefully Varying the concentrations of the reactants you will determine the effect each reactant has on the rate of the reaction and consequently the order of a quot 39 rrquot gi gaih 539 C 1 I P U Light J SOL FOE quotquotquotquotquotquot 399 1lt ltult1j Detector E E E EE E EEE E39E EEE39E39E EE E39E39E E ii I I I MATERIALS computer Vernier Colorimeter plastic cuvettes 103 PROCEDURE 1 2 3 Obtain and wear goggles Connect a Colorimeter to Channel 1 of the Vernier computer interface Start the Logger Pro program on your computer Open the file 3 Ob Crystal Violet from the Chemistry with Computers folder Set up and calibrate the Colorimeter a b c d e f g h i Prepare a blank by filling an empty cuvette 3 full with distilled water Place the blank in the cuvette slot of the Colorimeter and close the lid If your Colorimeter has a CAL button set the wavelength on the Colorimeter to 430 nm press the CAL button and proceed directly to Step 5 If your Colorimeter does not have a CAL button continue with this step to calibrate your Colorimeter Choose Calibrate P CH1 Colorimeter from the Experiment menu then click Tum the wavelength knob on the Colorimeter to the 0 T position Type 0 in the edit box When the displayed Voltage reading for Reading 1 stabilizes click Tum the knob of the Colorimeter to the Blue LED position 470 nm Type 100 in the edit box When the Voltage reading for Reading 2 stabilizes click then click Obtain the materials you will need to conduct this experiment a b C 01 Two 25 mL graduated cylinders Approximately 50 mL of 0020 M KI solution in a 100 mL beaker Approximately 50 mL of 0020 M FeCl3 solution in a separate 100 mL beaker CAUTION The FeCl3 solution in this experiment is prepared in 01 M HCl and should be handled with care Approximately 60 mL of distilled water in a third 100 mL beaker During this experiment you will conduct 5 trials This step describes the process for conducting the trials using the Trial 1 Volumes When you repeat this process use the correct Volume for each trial based on the table below b C 01 Trial FeC3 mL KI mL H20 mL 1 100 100 00 2 100 50 50 3 50 100 50 4 75 50 75 5 50 75 75 Measure 100 mL of FeCl3 solution using a graduated cylinder and pour it into a medium test tube Measure 100 mL of K1 solution using a graduated cylinder Prepare a clean cuvette Consider opening an online stopwatch to make sure that all measurements are started reproducibly at the same time after mixing Add the 100 mL of K1 solution to the test tube containing 100 mL of FeCl3 solution Cover the end of the test tube with your thumb and quickly invert to mix 104 10 f Within 15 seconds of mixing the two solutions fill the cuvette 3 full with the mixture Wipe the outside of the cuvette with a tissue place it in the Colorimeter and close the lid and begin collecting absorbance data The timing of this step is imperative to receiving useful data practice several times with water before attempting with the K1 and FeCl3 solutions Click to begin collecting absorbance data Data will be gathered for 2 minutes Observe the progress of the reaction in the beaker When the data collection is complete carefully remove the cuvette from the Colorimeter Dispose of the contents of the beaker and cuvette as directed Rinse and clean the beakers and the cuvette for the next trial a39 Examine the graph of the first trial On the toolbar select the Slope button 1 quotquot Slide the cursor to the initial time point This tool will determine the initial slope and thus approximate the initial rate of the reaction Record the slope as the initial rate of the Trial 1 reaction Repeat Steps 69 to conduct Trials 25 Add the solutions to your medium sized test tube in this order water followed by Fe3 followed by I When you complete Step 9 use the same technique to analyze Trials 25 that you used to analyze Trial 1 DATA ANALYSIS 1 Calculate the initial molar concentration of FeCl3 and K1 for each reaction and prepare a data table containing the concentrations of each reaction and the initial reaction rate 2 What is the order of the reaction in FeCl3 and K1 Write the rate law expression for the reaction 9 105 Kinetics Lab Report Your report for this lab should include the following sections Abstract Your abstract should be written individually Introduction Include why you did this experiment relevant background and general equations Data Include your data table with the initial concentrations of each reactant and the initial rate of each reaction Include one graph showing all of the trials overlaid on one another Results Report the order of the reaction with respect to each reactant State the rate law for the reaction Be sure to attach hand written sample calculations to the back of your report Discussion Discuss the experiment and any possible sources of error Explain how you determined the order of each reactant As part of your discussion answer the following question 1 Is it possible to calculate the rate constant k from your data If so calculate the rate constant If not explain why not 106 PreLab Chemical Equilibrium Finding a Constant Kc Part A Answer the following questions in your lab notebook be sure to show work for any calculations 1 Write the equilibrium constant expression for the experiment you will be studying this week 2 If the equilibrium values of Fe3 SCN and FeSCN2 are 95 X 104 M 36 X 104 M and 57 X 105 M respectively what is the value of Kc 3 Write the general form of the dilution equation 4 A solution is prepared by adding 18 mL of 0200 M FeNO33 and 2 mL of 00020 M KSCN Calculate the initial concentrations of Fe3 and SCNquot in the solution Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 107 Chemical Equilibrium Finding a Constant Kc The purpose of this lab is to experimentally determine the equilibrium constant KC for the following chemical reaction Fe3aq SCNaq lt gt FeSCN2aq ironIII thiocyanate thiocyanoironIII When Fe3quot39 and SCN are combined a dynamic equilibrium is established between these two ions and the FeSCN2quot39 ion In order to calculate the equilibrium constant Keq for the reaction it is necessary to know the concentrations of all the ions at equilibrium In this experiment four separate equilibrium systems or trials containing different concentrations of these three ions Fe3 SCN39 and FeSCN2 will be determined experimentally The Values for these equilibrium concentrations will be substituted into the equilibrium constant expression to see if Keq is indeed constant despite Varied initial concentrations for the reactants The Keq is determined by using the Law of Mass Action aAbBltgtcCdD This equation gives the equilibrium constant expression of Keq C1 D1dAr Bquot In order to determine the equilibrium concentrations for the three ions a standard solution needs to be prepared To prepare the standard solution a Very large concentration of Fe3quot39 will be added to a small initial concentration of SCN hereafter referred to as SCN39 1 The initial Fe3quot39 in the standard solution is 900 times larger than SCN39j According to LeChatelier39s principle which states that when a system in dynamic equilibrium is disturbed the system responds so as to minimize the disturbance and retum the system to a state of equilibrium This high initial concentration Fe3 ions on the left side of the equation forces the reaction far to the right using up nearly 100 of the initial SCN ions Using stoichiometry and the balanced equation it is assumed that for every mole of FeSCN2quot39 produced one mole of SCN is used up Thus since nearly all of the SCN39 ions are consumed in order to minimize the disturbance the product s concentration FeSCN2quot39std at equilibrium is assumed to be equal to the SCN Since the reaction produces the FeSCN2quot ions and this ion transmits the color red the solution s absorbance of blue light can be measured through the use of a colorimeter see Figure 1 Because the red solutions absorb blue light Very well the blue LED setting on the Colorimeter is used The computer interfaced Colorimeter measures the amount of blue light absorbed by the colored solutions absorbance A 108 BEEIquotS Law standard 1 m L 5 standard 2 E standard 3 E E Standards Euntztsntratinn uf lII39III39IlJllII39I Concentration mulx39L Figure 1 Figure 2 According to Beer s Law there is a direct relationship between a solution s concentration and its absorbance In this case the concentration is the FeSCN2 ion and the absorbance is blue light 470 nm In other words as the concentration of FeSCN2 increases so will the absorbance of blue light see Figure 2 The concentration of FeSCN2quot39 for any of the equilibrium systems trials 14 can be found by comparing the absorbance of each equilibrium system Agq to the absorbance of the standard solution Astd according to the following equation FesCN21 stdAstd F SCN2leq Aeq Since the concentration of FeSCN2quot Std is known and the all of the absorbances for the equilibrium solutions and the standard are measured and recorded all that needs to be done is to solve for the unknown Ae FeSCN2quot393q Q X FeSCN2Std Knowing the FeSCN2quot393q allows you to determine the concentrations of the other two ions at equilibrium For each mole of FeSCN2quot39 ions produced one less mole of Fe3quot39 and SCN ions will be found in the solution see the 11 ratio of coefficients in the equation on the previous page At equilibrium the Fe3quot39 and SCN39 can be determined according to the following equations F 3eq F 3i FesCN21 eq sCN Jeq semi FeSCN2quot393q Knowing the Values of Fe3quot39 q SClT3q and FeSCN2eq Wu can now calculate the Value of Kc the equilibrium constant 109 OBJECTIVE In this experiment you will determine the equilibrium constant Kc for the following chemical reaction Fe3aq SCN aq lt gt FeSCN2aq ironIII thiocyanate thiocyanoironIII MATERIALS Vemier Colorimeter 1 plastic cuvette ve 20 X 150 mm test tubes pipet bulb or pipet pump Serological pipet PROCEDURE 1 2 Obtain and wear goggles Label four 20 X 150 mm test tubes 14 Pour about 30 mL of 00020 M FeNO33 into a clean dry 100 mL beaker Pipet 50 mL of this solution into each of the four labeled test tubes Use a pipet pump or bulb to pipet all solutions CAUTION FeNO33 solutions in this experiment are prepared in 10 M HNO3 and should be handled with care Pour about 25 mL of the 00020 M KSCN into another clean dry 100 mL beaker Pipet 2 3 4 and 5 mL of this solution into Test Tubes 14 respectively Obtain about 25 mL of distilled water in a 100 mL beaker Then pipet 3 2 1 and 0 mL of distilled water into Test Tubes 14 respectively to bring the total volume of each test tube to 10 mL Mix each solution thoroughly with a stirring rod Be sure to clean and dry the stirring rod after each mixing Measure and record the temperature of one of the above solutions to use as the temperature for the equilibrium constant Kc Volumes added to each test tube are summarized below Test TUbe FeNO33 H20 Number mL mL mL 1 5 2 3 2 5 3 2 3 5 4 1 4 5 5 0 Prepare a standard solution of FeSCN2 by pipetting 18 mL of 0200 M FeNO33 into a 20 X 150 mm test tube labeled 5 Pipet 2 mL of 00020 M KSCN into the same test tube Stir thoroughly Connect the Colorimeter to the computer interface Prepare the computer for data collection by opening the file Lab 8 Equilibrium from the Chemistry 228 folder of LoggerPro Prepare a blank by filling a cuvette 34 full with distilled water To correctly use a Colorimeter cuvette remember o All cuvettes should be wiped clean and dry on the outside with a tissue 0 Handle cuvettes only by the top edge of the ribbed sides 110 o All solutions should be free of bubbles 0 Always position the cuvette with its reference mark facing toward the white reference mark at the top of the cuVette slot on the Colorimeter Calibrate the Colorimeter a Open the Colorimeter lid b Holding the cuVette by the upper edges place it in the cuVette slot of the Colorimeter Close the lid c If your Colorimeter has a CAL button Press the lt or gt button on the Colorimeter to select a wavelength of 470 nm Blue for this experiment Press the CAL button until the red LED begins to ash Then release the CAL button When the LED stops ashing the calibration is complete Proceed directly to Step 7 If your Colorimeter does not have a CAL button continue with this step to calibrate your Colorimeter First Calibration Point d Choose Calibrate gt CH1 Colorimeter T from the Experiment menu and then click e Tum the wavelength knob on the Colorimeter to the O T position f Type O in the edit box g When the displayed Voltage reading for Reading 1 stabilizes click Second Calibration Point h Tum the knob of the Colorimeter to the Blue LED position 470 nm i Type I00 in the edit box j When the displayed Voltage reading for Reading 2 stabilizes click then click You are now ready to collect absorbance data for the four equilibrium systems and the standard solution a Click to begin data collection b Empty the water from the cuvette Rinse it twice with 1 mL portions of the Test Tube 1 solution c Wipe the outside of the cuVette with a tissue and then place the cuvette in the Colorimeter After closing the lid wait for the absorbance Value displayed in the meter to stabilize Then click type I the trial number in edit box and press the ENTER key d Discard the cuVette contents as directed by your teacher Rinse the cuvette twice with the Test Tube 2 solution and fill the cuvette 34 full Follow the Step c procedure to find the absorbance of this solution Type 2 in the edit box and press ENTER e Repeat the Step d procedure to find the absorbance of the solutions in Test Tubes 3 4 and 5 the standard solution f From the table record the absorbance Values for each of the five trials in your data table g Dispose of all solutions as directed by your instructor 111 PROCESSING THE DATA 1 2 Write the KC expression for the reaction in the Data and Calculation table Calculate the initial concentration of Fe3 based on the dilution that results from adding KSCN solution and Water to the original 00020 M FeNO33 solution See Step 2 of the procedure for the Volume of each substance used in Trials 14 Calculate Fe3i using the equation FeNO33 mL 3 Fe 1 total mL x 00020 M This should be the same for all four test tubes Calculate the initial concentration of SClT based on its dilution by FeNO33 and Water KSCN mL SCN39i mtotal mL x 00020 M In Test Tube 1 SClTi 2 mL 10 mL00020 M 000040 M Calculate this for the other three test tubes FeSCN2eq is calculated using the formula FeSCN2eq 31 gtlt FeSCN2Std Astd where A6 and Astd are the absorbance Values for the equilibrium and standard test tubes respective y and FeSCN2Std 11000020 000020 M Calculate FeSCN2eq for each of the four trials Fe3eq Calculate the concentration of Fe3 at equilibrium for Trials 14 using the equation F33eq F 3i F3SCN2eq SCNeq Calculate the concentration of SCN at equilibrium for Trials 14 using the equation 2 SCNquoteq SClTi FeSCN eq Calculate KC for Trials 14 Be sure to show the KC expression and the Values substituted in for each of these calculations Using your four calculated KC Values determine an average Value for KC How constant Were your KC Values 112 Equilibrium Lab Report Your report for this lab should include the following sections Abstract Your abstract should be written individually Introduction Include why you did this experiment relevant background and general equations Data Include your data table with initial concentrations of each reactant for each trial Results Include a results table with calculated Keq values for each trial and an average value for Keq Be sure to attach hand written sample calculations to the back of your report Discussion Discuss the experiment and any possible sources of error In addition answer the following question as part of your report 1 How are you Keq values to each other Are they close enough to justify the assertion that an equilibrium constant is constant 2 What factors could have led to variations in Keq between trials 113 Le Chatelier s Principle in a Cobalt Complex A chemical system will eventually come to a dynamic state of equilibrium This state of equilibrium can be altered by adding some sort of stress to the reaction such as increasing a reactant or products concentration or by heating or cooling the reaction mixture Le Chatlier s principle states that wen a system in equilibrium is disturbed the composition of the system changes in a way to reduce or counteract the disturbance In this activity you will be monitoring the effect of disturbances of the equilibrium between two complex ions of cobalt Complex ions will be discussed in more detail during the third term but are formed when transition metals are bound to electron pair donors through coordinate covalent bonds CoCl4239aq 6H2O1 2 CoH2O6 24aq 4C1 q Hazards 6M HCl is very corrosive Exercise caution around solution and vapors Always make sure to add acid to less concentrated solutions rather than adding less concentrated solutions to the acid Sliver nitrate will stain skin and clothes exercise caution and be sure to wash your hands after the lab Always clean up any spills Disposal Cobalt chloride and silver nitrate as well as any excess acid must be properly disposed in the labeled waste jars Procedure 1 Prepare a hot bath by half filling a beaker with water and placing it on a hot plate Heat to boiling on medium heat Additionally prepare an icewater cold in a second beaker 2 Obtain 10 mL of a 02 M cobalt chloride solution Note the color 3 Obtain 30 mL of a 60 M HCl solution and slowly add drop wise to your cobalt chloride solution until vivid color change is observed Note the color 4 Divide the resulting solution amongst 4 small testtubes filling only halfway Do not overfill the testtubes about 5 mL in each Keep one as a control something that you can use to compare any color changes to 5 To one test tube add distilled water drop wise until the color changes 6 Keeping track of which test tube is which take a test tube from step 4 and the test tube generated in step 5 and place them in the hot bath for 2 minutes Note any color changes 7 Take the testtubes from step 6 and place in your ice bath for two minutes Note any color changes 8 To a test tube from step 4 add 01 M silver nitrate drop wise until a precipitate forms Note any color changes 114 Last Name First Name Color COC14 2aq C0H2O6l 2aq Step 3 Color change and observations Step 5 Color change and observations Step 6 Color change and observations Step 7 Color change and observations Step 8 Color Change and observations Questions 1 What was the effect of adding excess chloride ions Use Le Chatlier s principle and provide evidence 115 2 Based upon the heating and cooling of the two equilibrium mixtures propose if the reaction is endothermic or exothermic Use Le Chatlier s principle and provide evidence Would heat be considered as a reactant or product CoC14239aq 6H2O1 CoH2O6 24aq 4C1 q 3 How did the addition of silver nitrate affect the equilibrium if neither silver ions nor nitrate ions are in the equilibrium expression Use Le Chatlier s principle and provide evidence Additionally write a net ionic equation to describe the precipitation reaction 4 When perturbing the equilibrium with heating and cooling how many times do you think the equilibrium can be shifted before it stops Working Why How about modification of the equilibrium through changes in concentration 116 CH229 LABS Prelab Acid Rain Part A Answer the following questions in your lab notebook be sure to show your work for any calculations 1 Carbon dioxide CO2 reacts with water to produce carbonic acid H2CO3 Write the balanced chemical equations for this reaction and showing what happens when carbonic acid is dissolved in water What is the conjugate base of nitrous acid HNO2 Which is a stronger acid nitrous acid HNO2 or nitric acid HNO3 Which is a stronger base nitrite N02 or nitrate NO339 Describe the method you will use in this lab to generate the acids found in acid rain Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 117 Lab Acid Rain In this experiment you will observe the formation of four acids that occur in acid rain o carbonic acid H2CO3 o nitrous acid HNO2 o nitric acid HNO3 o sulfurous acid H2SO3 Carbonic acid occurs when carbon dioxide gas dissolves in rain droplets of unpolluted air 1 CO2g H2Ol j H2CO3aq Nitrous acid and nitric acid result from a common air pollutant nitrogen dioxide N02 Most of the nitrogen dioxide in our atmosphere is produced in automobile exhaust Nitrogen dioxide gas dissolves in rain drops and forms nitrous and nitric acid 2 2 NO2g H2Ol gt HNO2aq HNO3aq NO Sulfurous acid is produced from another air pollutant sulfur 2 dioxide S02 Most of the sulfur dioxide gas in the atmosphere results from burning coal that contains sulfur impurities Sulfur dioxide dissolves in rain drops and forms sulfurous acid 3 SO2g H2Ol gt H2SO3aq In the procedure outlined below you will first produce these three gases You will then bubble the gases through water producing the acids found in acid rain The acidity of the water will be monitored with a pH Sensor OBJECTIVES In this experiment you will o Generate three gaseous oxides CO2 S02 and N02 o Simulate the formation of acid rain by bubbling each of the three gases into water and producing three acidic solutions o Measure the pH of the three resulting acidic solutions to compare their relative strengths 118 MATERIALS 1 Beral pipet for HCl 3 Beral pipets with a 2 cm stem Vernier pH Sensor 3 Beral pipets with a 15 cm stem PROCEDURE 1 2 Wear gloves for this step Obtain another Beral pipet and label it HCl Squeeze the Obtain and wear goggles Obtain three short stem and three long stem Beral pipets as shown in Figure 1 Label the short stem pipets with the formula of the solid they will contain NaHCO3 NaNO2 and NaHSO3 Label the long stem pipets with the formula of the gas they will contain CO2 NO2 and SO2 You can use a 100 mL beaker to support the pipets pg Figure 1 Obtain a beaker containing solid NaHCO3 Squeeze the bulb of the pipet labeled NaHCO3 to expel the air and place the open end of the pipet into the solid NaHCO3 When you release the bulb solid NaHCO3 will be drawn up into the pipet Continue to draw solid into the pipet until there is enough to fill the curved end of the bulb as shown in Figure 1 Repeat the Step 3 procedure to add solid NaNO2 and NaHSO3 to the other two Beral pipets CAUTION Avoid inhaling dust from these solids Use a utility clamp to attach a 20 x 200 mm test tube to the ring stand Add about 4 mL of distilled water to the test tube Remove the pH Sensor from the pH 3 storage solution rinse it off with distilled water and place it into the distilled 0 water in the test tube Fi ure 2 Connect the pH Sensor to the computer interface Prepare the computer for data g collection by opening the le Exp 23 acid rain from the Chemistry with Computer folder of Lo ggerPro Calibrate the pH probe using 2 of the available calibration buffers Your TA will demonstrate this procedure bulb to expel some of the air and place the open end of the pipet into a beaker containing 10 M HCl When you release the bulb HCl will be drawn up into the pipet CAUTION HCl is a strong acid Gently hold the pipet with the stem pointing up so that HCl drops do not escape Insert the narrow stem of the HCl pipet into the larger opening of the pipet containing the solid NaHCO3 as shown in Figure 2 Gently squeeze the HCl pipet to add about 20 drops of HCl solution to the solid NaHCO3 When finished remove the HCl pipet and place it open side up in the 100 mL beaker Gently swirl the pipet that contains NaHCO3 and HCl Carbon dioxide CO2 is generated in this pipet and is heavier than air so it stays in the pipet Place the Beral pipet in the 100 mL beaker with the stem up to prevent spillage Squeeze all of the air from the bulb of the long stem pipet labeled CO2 Keep the bulb completely collapsed and insert the long stem of the pipet down into the gas generating pipet labeled NaHCO3 as shown in Figure 3 Be sure the tip of the long stem pipet remains above the liquid in the gas generating pipet Release the pressure on the bulb so that it draws gas up into it Store the gas generating pipet in the 100 mL beaker and invert the long stem pipet to keep the CO2 in until the next step 119 10 Insert the long stem pipet labeled CO2 into the test tube alongside the pH Sensor so that its tip extends into the water to the bottom of the test tube see Figure 4 11 To begin collecting data on the computer click After 15 seconds have elapsed gently squeeze the bulb of the pipet so that CO2 slowly bubbles up through the solution Use both hands to squeeze all of the gas from the bulb When data collection stops after 120 seconds examine the data in the table and determine the initial pH value before CO2 was added and the final pH value after CO2 was added and the pH stabilized To confirm these two values click the Statistics button and examine the minimum and maximum values in the pH box displayed on the graph Record the initial and final pH values in your lab notebook Close the Statistics box by clicking in the upper left corner of the box 12 Remove the pH Sensor from the test tube and rinse its tip thoroughly with distilled water and return it to the sensor storage solution Discard the contents of the test tube as directed by your TA Rinse the test tube thoroughly with tap water Add 4 mL of tap water to the test tube Place the pH Sensor in the test tube and check to see that the input display shows a pH that is about the same as the previous initial pH value If not rinse the test tube again and refill it 13 From the Experiment menu choose Store Latest Run This stores the data so it can be used later but it will be still be displayed while you do your second and third trials 16 Repeat the procedure in Steps 513 but this time adding HCl to the pipet containing solid NaHSO3 Sulfur dioxide S02 is generated in this pipet 17 Repeat the procedure in Steps 513 finally adding HCl to the pipet I containing solid NaNO2 Nitrogen dioxide N02 is generated in this pipet Leave all three gas generating pipets in the 100 mL beaker until Flgure 4 Step 18 18 When you are finished rinse the pH Sensor with distilled water and retum it to the sensor storage solution Clean and return the seven Beral pipets to the stockroom 19 Label all three curves by choosing Text Annotation from the Insert menu and typing carbon dioxide or nitrogen dioxide or sulfur dioxide in the edit box 20 Print copies of the graph with all three data sets displayed PROCESSING THE DATA For each of the three gases calculate the change in pH ApH by subtracting the initial pH from the final pH Record these values in the data table in your lab notebook 120 Acid Rain Lab Report Your report for this lab should include the following sections Abstract Provide final pH and change in pH for all three gases and identify the trend in acidity Introduction Say something about the effects of acid rain Data Include a table with the data from all three gases Results Calculate the change in pH for each of the gases and summarize in a table Discussion Discuss the experiment and any possible sources of error see question 6 below for assistance Answer the following questions as part of your discussion 1 In this experiment which gas caused the smallest drop change in pH 2 Which gas or gases caused the largest drop in pH 3 Coal from western states such as Montana and Wyoming is known to have a lower percentage of sulfur impurities than coal found in the eastem United States How would burning lowsulfur coal lower the level of acidity in rainfall Use specific information about gases and acids to answer the question 4 High temperatures in the automobile engine cause nitrogen and oxygen gases from the air to combine to form nitrogen oxides Which two acids in acid rain result from the nitrogen oxides in automobile exhaust 5 Which gas and resulting acid in this experiment would cause rainfall in unpolluted air to have a pH value less than 7 sometimes as low as 56 All three gases are produced by man but one occurs naturally at relatively high concentrations 6 Look up the Ka values for each acid Which acid is the weakest Which is the strongest 121 Prelab Acid Dissociation Constant K Part A Answer the following questions in your lab notebook be sure to show work for any calculations 1 What would the pH of a 010 M solution of NaOH be in theory 2 What would be the pH of a 010 M solution of HCl 3 Write the equilibrium constant expression Ka for the dissociation of acetic acid HC2H3O2 4 What would be the predicted pH of a 10 M solution of HC2H3O2 5 Determine the volume in mL of 200 M HC2H3O2 required to prepare 100 mL of a 030 M HC2H3O2 solution Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 122 Acid Dissociation Constant Ka Acetic Acid HC2H3O2 is a weak acid that dissociates according to the balanced chemical equation HC2H3O28q lt gt Haq C2H3O28q In this experiment you will experimentally determine the dissociation constant Ka for acetic acid starting with solutions of different initial concentrations OBJECTIVES In this experiment you will o Gain experience mixing solutions of specified concentration o Experimentally determine the dissociation constant Ka of an acid o Investigate the effect of initial solution concentration on the extent of dissociation MATERIALS Vemier pH Sensor pipets pipet bulb 2 x 100 mL volumetric asks Figure 1 P ROC E DU R E 1 Obtain and wear safety goggles 2 Put approximately 25 mL of distilled water into a 50 mL volumetric ask 3 Your TA will assign each group two different concentrations of HC2H3O2 Calculate the volume of a 20 M HC2H3O2 stock solution necessary to make 50 mL of each solution you were assigned Pipet the required volume of 200 M acetic acid into the volumetric ask CAUTION Use care when handling the acetic acid It can cause painful burns if it comes in contact with your skin or gets into your eyes Fill the ask with distilled water to the 50 mL mark To prevent overshooting the mark use a wash bottle filled with distilled water or a dropper for the last few mL Mix thoroughly 123 4 Use a utility clamp to secure a pH Sensor to a ring stand as shown in Figure 1 5 Connect the probe to the computer interface Prepare the computer for data collection by opening the le Exp 27 Acid Dissociation Ka from the Chemistry w Computer folder of Lo ggerPro 6 Determine the pH of your solution as follows o Use about 40 mL of distilled water in a 100 mL beaker to rinse the pH Sensor o Pour about 20 mL of your first assigned solution into a clean 100 mL beaker and use it to thoroughly rinse the sensor o Use the remaining 30 mL portion to determine pH Swirl the solution vigorously Note Readings may drift without proper swirling Record the measured pH reading in the data table in your lab notebook o When done place the pH Sensor in distilled water 7 Repeat the procedure for your second assigned solution PROCESSING THE DATA 1 Calculate the Heq from the pH Values for each solution 2 Use the obtained Value for Heq and the equation HC2H3O28q T H8q C2H3O28q to construct an ICE table and determine C2H3O2eq and HC2H3O2eq 3 Substitute these equilibrium concentrations into the K3 expression for HC2H3O2 4 Compare your results with those of other students 124 Acid Dissociation Lab Report Your lab report should include the following sections Abstract Report your two values of Ka and that obtained by the class as a whole Introduction Distinguish between weak acids and strong acids and give a literature value and citation for the acid dissociation constant of acetic acid Data Include the concentrations of the two HC2H3O2 solutions you made and the volume of the stock solution and water used to make the solutions Include a copy of the ICE table used for each concentration of HC2H3O2 Results Report your calculated Ka value for each concentration used and an average Ka Compare your experimentally determined Ka with the accepted value at 25 C 18 X 105 and calculate the percent error Calculate the percent dissociation of acetic acid for each concentration used Discussion Discuss the experiment and any possible sources of error Answer the following questions as part of your discussion 1 What effect does initial HC2H3O2 concentration seem to have on K3 2 What effect does initial HC2H3O2 concentration seem to have on the percent dissociation 3 In the past many students have listed that the accidental addition of too much acetic acid contributed greatly to the difference between the experimental value and the accepted value Suppose that Student A was supposed to make a 018 M solution by diluting 90 mL of 20 M acetic acid to 1000 mL The expected pH for this solution is 274 The Ka of acetic acid is 18 x 105 a What would be the expected pH if a Student A accidentally diluted 91 mL instead of 90 mL of the acid to 100 mL b If student A measured the above calculated pH what would be the resultant Ka of acetic acid given that they expected the acid to have an initial concentration of 018M 125 Prelab Titration of a Diprotic Acid Identifying an Unknown Part A Answer the following questions in your lab notebook be sure to show work for any calculations 1 What is a diprotic acid Give an example not found below in the text for this experiment 2 Give the balanced chemical reaction for the titration of a generic diprotic acid H2X with potassium hydroxide 3 When titrating 500 mL of 010 M H2SO4 with 010 M NaOH how many mL of NaOH will you have added to reach the 1 equivalence point 4 A student completes a titration of an unknown diprotic acid In this experiment 079 g of the acid is dissolved in 2500 mL of water It requires 1348 mL of 10 M NaOH to reach the second equivalence point What is the molar mass of the acid Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 126 Lab Titration of a Diprotic Acid Identifying an Unknown A diprotic acid is an acid that yields two H ions per acid molecule Examples of diprotic acids are sulfuric acid H2SO4 and carbonic acid H2CO3 A diprotic acid dissociates in water in two stages 1 H2Xaq lt gt Haq HX aq lt2 HX ltaqgt lt gt Hltaqgt X2 ltaqgt Because of the successive dissociations titration curves of diprotic acids can have two equivalence points as shown in Figure 1 The equations for the acid base reactions occurring between a diprotic acid H2X and sodium hydroxide base NaOH are from the beginning to the first equivalence point 3 HQX NaOH ltgt NaHX H20 from the first to the second equivalence point pH 4 NaHX NaOH lt gt Na2X H20 from the beginning of the reaction through the second equivalence point net reaction I I 5 HQX 2 NaOH lt gt Na2X 2 H20 i l 1st Equivalence Point 2nd Equivalence Point Volume NaOH At the first equivalence point all H ions from the first dissociation have reacted with NaOH base At Flgure 1 the second equivalence point all H ions from both reactions have reacted twice as many as at the first equivalence point Therefore the volume of NaOH added at the second equivalence point is exactly twice that of the first equivalence point see Equations 3 and 5 The primary purpose of this experiment is to identify an unknown diprotic acid by finding its molecular weight A known mass of a diprotic acid is titrated with NaOH solution of known concentration Molecular weight or molar mass is found in gmole of the diprotic acid Weighing the original sample of acid will tell you its mass in grams Moles can be determined from the volume of NaOH titrant needed to reach the first equivalence point The volume and the concentration of NaOH titrant are used to calculate moles of NaOH Moles of unknown acid equal moles of NaOH at the first equivalence point see Equation 3 Once grams and moles of the diprotic acid are known molecular weight can be calculated in gmole Molecular weight determination is a common way of identifying an unknown substance in chemistry You may use either the first or second equivalence point to calculate molecular weight or both The first is somewhat easier because moles of NaOH are equal to moles of H2X see Equation 3 If the second equivalence point is more clearly defined on the titration curve however simply divide its NaOH volume by 2 to confirm the first equivalence point or from Equation 5 use the ratio 1 mole H2X 2 mol NaOH 127 OBJECTIVE In this experiment you will identify an unknown diprotic acid by finding its molecular weight MATERIALS Vernier pH Sensor 50 mL buret Stir plate Magnetic stir bar PROCEDURE 1 Obtain and wear goggles 2 Weigh out about 0120 g of the unknown diprotic acid on a piece of 4 Add 6 drops of the indicator mixture The indicator mixture contains weighing paper Record the mass to the nearest 0001 g in the data table in your lab notebook Transfer the unknown acid to a 250 mL beaker and dissolve in 100 mL of distilled water CAUTION Handle the solid acid and its solution with care Acids can harm your eyes skin and respiratory tract a mixture of bromocresol green and phenol red Use a utility clamp to suspend a pH Sensor on a ring stand as shown here Position the pH Sensor in the diprotic acid solution and adjust its position toward the outside of the beaker so it will be easier to stir the solution with a magnetic stir bar without striking the sensor Get the stir bar spinning rapidly but smoothly and leave it on Obtain approximately 60 mL of 01 M NaOH solution in a 250 mL beaker Obtain a 50 mL buret and rinse the buret with a few mL of the 01 M NaOH solution Record the precise concentration of the NaOH solution in the data table in your lab notebook Use a utility clamp to attach the buret to the ring stand Fill the buret a little above the 000 mL level of the buret Drain a small amount of NaOH solution into a waste beaker so it fills the buret tip and leaves the NaOH close to but below the 000 mL level of the buret Be sure to record the actual buret reading ALL BURET READINGS NEED TO BE RECORDED TO TWO DECIMAL PLACES Dispose of the waste solution from this step in the waste jar as directed by your teacher CAUTION Sodium hydroxide solution is caustic Avoid spilling it on your skin or clothing Connect the pH Sensor to the computer interface Prepare the computer for data collection by opening the file Exp 24a Acid Base Titration from the Chemistry w Computer folder of LoggerPro You are now ready to begin the titration This process goes faster if one person manipulates and reads the buret while another person operates the computer and enters buret readings a Before adding NaOH titrant click and monitor the pH for 510 seconds Once the pH has stabilized click In the edit box type in the initial buret reading and press ENTER to store the first data pair for this experiment b Add enough NaOH to raise the pH by about 020 units When the pH stabilizes again click In the edit box type the current buret reading to the nearest 001 mL Press ENTER You have now saved the second data pair for the experiment c Continue adding NaOH solution in increments that raise the pH about 020 units and enter the buret reading after each addition Proceed in this manner until the pH is 35 d When pH 35 is reached change to 2drop increments Enter the buret reading after each increment Additionally note any change in the color of the solution 128 8 9 e After pH 45 is reached again add larger increments that raise the pH by about 020 units and enter the buret reading after each addition Continue in this manner until a pH of 75 is reached f When pH 75 is reached change back to 2 drop increments Enter the buret reading after each increment g When pH 10 is reached again add larger increments that raise the pH by 020 units Enter the buret reading after each increment Continue in this manner until you reach a pH of 1 1 When you have finished collecting data click Dispose of the beaker contents in the waste jar as directed by your TA Print a copy of the table Then print a copy of the graph PROCESSING THE DA TA 1 On your printed graph one of the two equivalence points is usually more clearly defined than the other the twodrop increments near the equivalence points frequently result in larger increases in pH a steeper slope at one equivalence point than the other Indicate the more clearly defined equivalence point first or second in your data table Use your graph and data table to determine the volume of NaOH titrant used for the equivalence point you selected in Step 1 To do so examine the data to find the largest increase in pH values during the 2 drop additions of NaOH Find the NaOH volume just before this jump Then find the NaOH volume after the largest pH jump Identify both of these data pairs and record them Determine the volume of NaOH added at the equivalence point you selected in Step 1 To do this add the two NaOH volumes determined in Step 2 and divide by two For example 1234 1244 T 1239 mL Calculate the number of moles of NaOH used at the equivalence point you selected in Step 1 Determine the number of moles of the diprotic acid H2X Use Equation 3 or Equation 5 to obtain the ratio of moles of H2X to moles of NaOH depending on which equivalence point you selected in Step 1 Using the mass of diprotic acid you measured out in Step 1 of the procedure calculate the molecular weight of the diprotic acid in gmol From the following list of five diprotic acids identify your unknown diprotic acid Diprotic Acid Formula Molecular weight Oxalic Acid H2C2O4 90 Malonic Acid H2C3H2O4 104 Maleic AC1d H2C4H2O4 116 Malic Acid H2C4H4O5 134 Tartaric Acid H2C4H4O6 150 Determine the percent error for your molecular weight value in Step 6 129 9 10 11 For the alternate equivalence point the one you did not use in Step 1 use your graph and data table to determine the volume of NaOH titrant used Examine the data to find the largest increase in pH values during the 2drop additions of NaOH Find the NaOH volume just before and after this jump Underline both of these data pairs on the printed data table and record them in the Data and Calculations table Note Dividing or multiplying the other equivalence point volume by two may help you confirm that you have selected the correct two data pairs in this step Determine the volume of NaOH added at the alternate equivalence point using the same method you used in Step 3 On your printed graph clearly specify the position of the equivalence point volumes you determined in Steps 3 and 10 using dotted reference lines like those in Figure 1 Specify the NaOH volume of each equivalence point on the horizontal axis of the graph 130 Extension Using a halftitration method it is possible to determine the acid dissociation constants K31 and K32 for the two dissociations of the diprotic acid in this experiment The K3 expressions for the first and second dissociations from Equations 1 and 2 are HHX39 K HX239 a1 H2X 9 HX The first halftitration point occurs when onehalf of the H ions in the first dissociation have been titrated with NaOH so that H2X HX Similarly the second halftitration point occurs when onehalf of the H ions in the second dissociation have been titrated with NaOH so that HX X2 Substituting H2X for HX in the K31 expression and HX for X2 in the K32 expression the following are obtained Kai H 39 Ka2 H 39 Taking the baseten log of both sides of each equation logK31 logH logK32 logH Thus the pH value at the first halftitration volume Point 1 in Figure 2 is equal to the pK31 value The first halftitration point volume can be found by dividing the first equivalence point volume by two Similarly the pH value at the second titration point pH is equal to the pK32 value The second halftitration volume Point 2 in Figure 2 is midway between rgtK the first and second equivalence point volumes 1st g EP and 2nd EP Use the method described below 1St39EP M H to determine the K31 and K32 values for the diprotic Volume NaOH acid you identified in this experiment Figure 2 1 Determine the precise NaOH volume for the rst halftitration point using onehalf of the first equivalence point volume determined in Step 2 or Step 9 of Processing the Data Then determine the precise NaOH volume of the second halftitration point halfway between the first and second equivalence points 2 On your graph of the titration curve draw reference lines similar to those shown in Figure 2 Start with the first halftitration point volume Point 1 and the second halftitration point volume Point 2 Determine the pH values on the vertical axis that correspond to each of these volumes Estimate these two pH values to the nearest 01 pH unit These values are the pK31 and pK32 values respectively Note See if there are volume values in your data table similar to either of the halftitration volumes in Step 1 If so use their pH values to confirm your estimates of pK31 and pK32 from the graph 3 From the pK31 and pK32 values you obtained in the previous step calculate the K31 and K32 values for the two dissociations of the diprotic acid 131 EQUIVALENCE POINT DETERMINATION An Alternate Method An alternate way of determining the precise equivalence point of the titration is to take the first and second derivatives of the pH volume data The equivalence point volume corresponds to the peak maximum value of the first derivative plot and to the volume where the second derivative equals zero on the second derivative plot 1 View the first derivative graph ApHAvol by clicking the on the vertical aXis label pH and choose First Divative You may need to autoscale the new graph by clicking the Autoscale button c 2 View the second derivative graph A2pHAvolz by clicking on the vertical aXis label and choosing Second Derivative In Method 2 view the second derivative on Page 3 by clicking on the Next Page button 132 Titration of a Diprotic Acid Lab Report You will write a complete lab report for this lab which will contain the following sections Abstract State the molar mass and which of the unknown diprotic acids you were given and what you found for the two Ka values Introduction Data Introduce the major concepts in this lab titration titration curves equivalence point diprotic acids and Ka Explain how you will make use of these concepts to determine the identity of your unknown acid Include equations where necessary Include in your report the recorded concentration of base used for the titration the mass of the unknown used Additionally summarize the relevant volume and pH data collected during your titration include data for the initial reading at each 12 equivalence point and at each equivalence point do not tum in your raw data it is summarized in the titration curve you are also to cite and attach Results Include in your report a copy of the titration curve the first derivative plot and the second derivative plot Tabulate your results for the calculated moles of acid the molar mass of the acid the identity of the acid and the percent error in your calculated molar mass for both equivalence points AND for both methods of determining equivalence point Include the determined values of Kal and Kag for your acid Attach your hand written calculations to your report Discussion Discuss the experiment and any possible sources of error Which data analysis method using the indicators or using the Venier pH data do you feel gave you better or more easy to interpret results Justify your answer How sure are you in the identification of your unknown Use both your Ka values and molecular weight to identify the appropriate acid Question When the pH of the solution equals the pK of an indicator the solution will have an intermediate color Estimate the pK of both indicators bromocresol green is the indicator that made the transition in the acidic region of the titration 133 Prelab Buffers Part A Answer the following questions in your lab notebook be sure to show work for any calculations 1 Buffer A Calculate the mass of solid sodium acetate required to mix with 500 mL of 01 M acetic acid to prepare a pH 4 buffer The Ka of acetic acid is 18 x 105 2 Buffer B Calculate the mass of solid sodium acetate required to mix with 500 mL of 10 M acetic acid to prepare a pH 4 buffer The Ka of acetic acid is 18 x 105 3 Write a reaction to show how a sodium acetateacetic acid buffer would respond to a small amount of added strong acid 4 Write a reaction to show how a sodium acetateacetic acid buffer would respond to a small amount of added strong base Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 134 Lab Buffers A buffer is a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid A buffer s function is to absorb small amounts of acids H or H3O ions or bases OH ions so that the pH of the system changes by a smaller amount than it would with most other solutions In many systems buffers are critical Blood plasma a natural example in humans is a bicarbonate buffer that keeps the pH of blood between 72 and 76 By design a buffer is an equilibrium system For example a buffer can be prepared with nitrous acid HNO2 The weak acid establishes an aqueous equilibrium as shown below HNO2 aq lt gt H aq N02 aq The equilibrium constant expression is shown below K H1N0 3 HNO2 To prepare a buffer system with nitrous acid a comparable amount of the conjugate base is added such as sodium nitrite NaNO2 The resulting system is a mixture of HNO2 and N02 ions The nitrous acid molecule will neutralize hydroxide ions and the nitrite ion the conjugate will neutralize hydronium ions and in each case the product will be one of the original conjugates resulting in a solution that is similar to the original one A Variation of the equilibrium expression above called the Henderson Hasselbalch equation is a useful reference in preparing a buffer solution For our nitrous acid sodium nitrate buffer example the Henderson Hasselbalch equation is shown below NO pH pKa Hogg HNO2 The pH range in which a buffer solution is effective is generally considered to be il of the pKa corresponding to ten fold excesses of either the acid over the conjugate base or Vice Versa In this experiment you will use the Henderson Hasselbalch equation to determine the amount of acetic acid and sodium acetate needed to prepare two acidic buffer solutions You will then prepare the buffers and test their buffer capacities by adding solutions of NaOH and HCl while monitoring the pH OBJECTIVES In this experiment you will o Prepare and test two acid buffer solutions o Determine the buffer capacity of the prepared buffers 135 MATERIALS Vernier pH Sensor two 50 mL burets PROCEDURE Part I Prepare and Test Buffer Solution A 1 Obtain and wear goggles 2 Use your calculations from the PreLab Exercise to prepare 50 mL of Buffer A Weigh out the precise mass of sodium acetate and dissolve it in 500 mL of 01 M acetic acid solution 3 Set up two burets buret clamps and ring stand see Figure 1 Rinse and fill one buret with 05 M NaOH solution Rinse and fill the second buret with 05 M HCl solution CAUTION Sodium hydroxide solution is caustic Avoid spilling it on your skin or clothing Handle the hydrochloric acid with care It can cause painful burns if it comes in contact with the skin 4 Use a graduated cylinder to measure out 100 mL of the Buffer A solution into a 250 mL beaker and add 15 mL of distilled water You will stir with a stirring rod during the testing 5 Connect a pH Sensor to Channel 1 of the Vernier computer interface Connect the interface to the computer using the proper interface cable 6 Calibrate the pH probe using two of the calibration buffers Your TA will demonstrate this 7 Suspend the pH Sensor in the pH 4 buffer solution as shown in Figure 1 8 Start the Logger Pro program on your computer Open the file 19 Buffers from the Advanced Chemistry with Vernier folder OIOIOIIIOIIIOIOOIlOQl IOi0llOlIlOlamp0gl0l00 Figure 1 9 You are now ready to test Buffer A Once you verify the initial pH of the buffer you will slowly and carefully add 05 M NaOH solution to the buffer solution a Take an initial pH reading of the buffer solution Click and monitor pH for 510 seconds Once the displayed pH reading has stabilized click In the edit box type the starting volume on the buret Press the ENTER key to store the first data pair 136 10 11 Record the initial pH value NOTE if the initial pH is not Within 03 pH units of 40 you should remake the buffer and begin again b Add a small amount of the NaOH solution up to 030 mL When the pH stabilizes click Enter the current buret reading and press ENTER to store the second data pair c Continue adding the NaOH solution in small increments that raise the pH consistently and enter the buret reading after each increment Your goal is to raise the pH of the buffer by precisely 2 pH units d When the pH of the buffer solution is precisely 2 units greater than the initial reading continue to add the NaOH solution in small increments until you have reached and passed the equivalence point of the titration The pH will begin to rise rapidly at the equivalence point e Click Print a copy of the first trial Dispose of the reaction mixture in the Waste jar as directed Rinse the pH sensor with distilled water in preparation for the second titration Repeat Steps 7 and 8 using a fresh 100 mL sample of the Buffer A solution For the second trial repeat using the sodium hydroxide For the third trial titrate the buffer with 05 M HCl solution Carefully add HCl in small increments until the pH of the solution has been lowered by precisely 2 units or no significant change continues to occur Record the volume of HCl that was used There is no need to print a copy of the graph but either print the data table or copy the data into your lab notebook Part II Prepare and Test Buffer Solution B 12 13 Use your calculations from the PreLab Exercise to prepare 50 mL of Buffer B Weigh out the precise mass of sodium acetate and dissolve it in 500 mL of 10 M acetic acid solution If necessary refill the burets of NaOH and HCl solution Use a graduated cylinder to measure out 100 mL of the Buffer B solution Repeat the necessary steps to test Buffer B in a manner similar to the Part I trials Print a copy of your graph of the titration using the NaOH solution Record the volume of HCl that was used to lower the pH of Buffer B by 2 units or until no significant change continues to occur 137 Buffers Lab Report Your lab report should include the following sections Abstract Report the buffer capacity defined below for each of your buffers Introduction Make sure you include balanced chemical equations that explain how your acetic acid acetate buffer reacts with a strong acid and with a strong base Data Include all collected data in the form of a data table This does not mean you should attach your raw data to your report Summarize the data to include those data points that are used to calculate the buffer capacity Results Include the graph of pH VS Volume of NaOH added for both buffer A and B Buffer capacity has a rather loose definition yet it is an important property of buffers A commonly seen de nition of buffer capacity is The amount of H or OHquot that can be neutralized before the pH changes by 1 pH unit Use your data to determine the buffer capacity of Buffer A and Buffer B and include this in the results section Use the equation below to express buffer capacity Buffer capacity moles of acid or base added change in pH Discussion Discuss the experiment and any possible sources of error Answer the following questions as part of your discussion 1 Say for example that you had prepared a Buffer C in which you mixed 8203 g of sodium acetate NaC2H3O2 with 1000 mL of 10 M acetic acid a What would be the initial pH of Buffer C b If you add 50 mL of 05 M NaOH solution to 200 mL each of Buffer B and Buffer C which buffer s pH would change less Explain 138 2 If you wanted to carry out an experiment at physiological pH 74 could you suggest an appropriate buffer system 3 Why were the buffers asymmetric that is they seemed to handle added sodium hydroxide with relatively small changes in the pH While small quantities of HCl caused the pH to decrease substantially 139 Prelab Determination of the KS1 of Calcium Hydroxide Part A Answer the following questions in your lab notebook be sure to show your work for any calculations 1 Write the molecular balanced chemical equation and the net ionic equation for the reaction between CaOH2aq and HClaq 2 What is the mole ratio between OH39 and H39 between Ca2 and H39 3 The molar solubility of a slightly soluble ionic compound M2X3 is 28 X 106 M Determine the Value of Ksp 4 Which of the saturated solutions below would have the highest OH a MOH2 Ksp 425 x 10 b MOH2 Ksp 739 x 10394 c MOH2 Ksp 264 x 10 3 d MOH2 Ksp 852 x 10 5 Which of the saturated solutions shown in question 4 would have the lowest pH Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 140 Lab Determination of the K5 of Calcium Hydroxide From Advanced Chemistry with Vernier Vernier Software amp Technology 2004 INTRODUCTION Calcium hydroxide is a strong base that is sparingly soluble in water In solution it completely dissociates into ions as represented by the following balanced chemical equation CaOH2 S lt gt Ca2 aq 2OH aq The solubility product expression describes in mathematical terms the equilibrium that is established between the solid substance and its dissolved ions in an aqueous system The equilibrium expression for calcium hydroxide is shown below K Ca2OH392 Equation 1 The constant that quantifies a substance s solubility in water is called the solubility product Ksp All compounds even highly soluble ones like sodium chloride have a Ksp However the Ksp of a compound is usually only considered in cases where the compound is slightly soluble and thus the concentration of solvated ions is small The primary objective in this experiment is to test a saturated solution of calcium hydroxide and use your observations and measurements to calculate the Ksp of the compound You will do this by three methods The first will be to determine the concentration of CaOH2 by titrating with a standardized hydrochloric acid solution By determining the molar concentration of dissolved hydroxide ions in the saturated CaOH2 solution and assuming they all come from calcium hydroxide you will have the necessary information to calculate the Ksp The second method will be to measure the pH of the saturated solution and calculate the concentration of the OH39aq ions and use that information to determine the Ksp The third method will use gravimetric analysis to determine the quantity of CaOH2 dissolved in a known volume of solution The mass of dissolved solid can be used to determine the solubility and thus the Ksp of CaOH2 OBJECTIVES In this experiment you will use three separate methods to determine the Ksp of calcium hydroxide and will compare the ease of use and reliability of the three 141 MATERIALS Vernier pH Probe Buret PROCEDURE CAUTION Calcium hydroxide solution is caustic Avoid spilling it on your skin or clothing 1 Obtain and wear goggles 2 Obtain about 70 mL of a saturated calcium hydroxide solution 3 Set up a ring stand ring filter funnel and filter paper as demonstrated by your instructor Filter your sample of CaOH2 solution into a clean beaker Method 1 Gravimetric Determination begin one lab period in advance 1 Using a 25 mL graduated cylinder transfer exactly 20 mL of the filtered solution into a pre weighed and labeled 100 mL beaker The beaker must be labeled with your section number and your drawer number 2 Place the beaker in the warming oven until the end of the lab period 3 After the drying period determine the mass of the beaker plus dry calcium hydroxide and by difference the mass of the calcium hydroxide Method 2 Determination by pH 1 Connect a pH Sensor to Channel 1 of the Vernier computer interface Connect the interface to the computer using the proper interface cable 2 Start the Logger Pro program on your computer and allow the default program to open 3 Calibrate the pH probe following this procedure 0 Use the 2point calibration option of the Vernier data collection program Rinse the tip of the electrode in distilled water Place the electrode into one of the buffer solutions eg pH 4 When the voltage reading displayed on the computer or calculator screen stabilizes enter a pH value 4 0 For the next calibration point rinse the electrode and place it into a second buffer solution eg pH 7 When the displayed voltage stabilizes enter a pH value 7 0 Rinse the electrode with distilled water It is now ready to beplaced in the sample to be measured 4 Obtain about 25 mL of the filtered solution in a 100 mL beaker and measure and record the pH of the filtered solution in your lab notebook Retain this solution in case you need it for the third method below If not dispose of it in the waste jar provided 142 Method 3 Determination by Titration 1 Using a 10 mL volumetric pipette measure exactly 10 mL of the filtered solution into a 250 mL beaker 2 Add 3 drops of the methyl red indicator solution The solution will turn yellow 3 Obtain about 70 mL of 0050 M HCl solution 4 Connect a buret to the ring stand Rinse the buret with 10 mL of the acid before filling it with the 00500 M HCl solution 5 Record the initial volume of HCl in your buret to 001 mL as usual 6 Using a glass stirring rod to stir the solution semi rapidly titrate your CaOH2 sample with the HCl until the yellow color just begins to tum into an orange color Begin adding the HCl dropwise until the solution just turns red in color 7 Record the final volume of HCl in your buret and determine the volume of HCl used in the titration by difference 8 Dispose of the reaction mixture in the labeled waste bottle in the fume hood 9 Repeat the steps above to titrate a second sample of the filtered CaOH2 solution 10 If the volume of HCl used in the two titrations differs by more than 10 do a third titration PROCESSING THE DATA Calculations Determination of Ksp using the pH of the solution Use the pH to determine the OH With the balanced chemical equation defining the dissolution of CaOH2 use the OH to determine the Ca2 assuming that both ions came from calcium hydroxide Substitute the values into equation 1 to determine the Ksp Gravimetric determination of Ksp Determine the mass of dissolved solid Calculate the solubility of the solution M using the mass of dissolved solid the volume of solution used in the analysis and the molar mass of calcium hydroxide Substitute the molar solubility into a version of equation 1 Hint use an ICE table to determine the Ksp Determination of Ksp through titration 143 Using the concentration and Volume of HC1 used determine the OH39 in the saturated solution Again use the balanced chemical equation to link the OH to Ca2 and assume calcium hydroxide was the only source of both Substitute the concentrations into equation 1 to determine the Ksp 144 Determination of the K5 of Calcium Hydroxide Lab Report Your report for this lab should include the following sections Abstract Give the Ksp for the three methods and suggest which method seems most reliable g if all seem equally valid say so and average the three results to obtain a best value In either case provide the percent error Introduction Give the equations and a short description of the use of equilibrium constant expressions in calculating solubility Data Include 3 data tables one for each method used to determine the Ksp Results Calculate the values of Ksp for each method Construct a data table summarizing the results Provide an average of the results Discussion Use the data in Appendix II of your test book to determine the accepted value for the Ksp Calculate your percent error with respect to each result Speak about which method provided you the most accurate results and why you think that particular method was more accurate and the others were not Or conclude that all three seemed to work under what conditions would this seem to be a valid conclusion In addition answer the following question as part of your discussion 1 Predict how each of the following mistakes would affect the value of the measured Ksp and provide reasoning for your response a The buret was inadvertently left wet with water from cleaning The measured Ksp would be lower than the true value The measured Ksp would be higher than the true value The measured Ksp would be same as the true value b Using phenolphthalein as the indicator instead of methyl red The measured Ksp would be lower than the true value The measured Ksp would be higher than the true value The measured Ksp would be same as the true value 145 Prelab Thermodynamics of the Solubility of Potassium Nitrate Part A Answer the following questions in your lab notebook be sure to show work for any calculations 1 2 What is AG What does the sign of AG tell you about the spontaneity of a reaction A researcher wants to make a solution of AgCl and water at 75 C For solid AgCl at 75 C Ksp 15 x 105 a Calculate the free energy change associated with making a saturated solution of AgCl in water at 75 C b How many grams of AgCl will dissolve in 10 L of water at 75 C The Ksp for Ag2C1 O4 is 90 x 1039 If 200 mL of 00050 M AgNO3 is combined with 300 mL of 00020 M K2CrO4 will a precipitate form Do you expect the dissolution of KNO3 to be endothermic or exothermic Use Appendix II in your text to calculate AHdisS01uti0n Do you expect the dissolution of KNO3 to have a positive or negative AS Use Appendix II in your text to calculate ASdiss01uti0n Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 146 Lab Thermodynamics of the Solubility of Potassium Nitrate In this experiment you will measure the solubility of KNO3 as a function of temperature The data collected will be used to determine the Ksp enthalpy entropy and free energy of dissolution When a salt dissolves in water it will dissociate into ions In aqueous solution potassium nitrate KNO3 dissociates according to the following reaction KN03lts Kltaq N0339ltaq As the concentration of dissolved K and N03quot increases the rate at which the ions will recombine into solid potassium nitrate KNO3 also increases At one set of ion concentrations the rate of dissolution will equal the rate of precipitation At this point the reaction is said to be at equilibrium The solution is now considered saturated An equilibrium expression Ksp for this process is shown in equation 1 K51 KNO339 Equation 1 The value for Ksp is characteristic of each compound and changes with the temperature Thermodynamics may be used to understand the energy changes that occur when a salt dissolves in water The energy difference between the solid salt and its dissolved ions is known as the enthalpy change AH and the relative disorder of the dissolved ions is an indication of the entropy change AS A positive enthalpy change will occur if heat must be added to dissolve the salt in water The enthalpy change will be negative if the dissolution process releases heat The entropy change for a solid salt dissolving in water will always be positive because the dissolved ions possess more disorder than a solid crystalline salt The free energy change AG for a process will indicate if the process is spontaneous as written reactants going to products A negative value indicates that the process is spontaneous while a positive value denotes a nonspontaneous process The Gibbs Helmholtz equation shown in Equation 2 is a mathematical expression that relates changes in free energy enthalpy and entropy AG AH TAS Equation 2 AG can also be expressed in terms of Ksp equation 3 AG RTlnKSp Equation 3 By combining Equations 2 and 3 it is possible to derive an equation that relates K513 and the Kelvin temperature to the values associated with AH and AS equation 4 lnKSp equation 4 147 By plotting lnKsp versus 1T we get a line where the slope of the line is AHR and the y intercept is ASR R is the ideal gas constant 8314 J mol391K4 The enthalpy and entropy of dissolution can be determined by evaluating the temperature dependence of Ksp OBJECTIVES In this experiment you will o Determine the solubility of KNO3 as a function of temperature o Use the solubility data to determine the Ksp for the dissolution of KNO3 o Use the data and Equations 4 and 2 to calculate AG AH and AS for the dissolution process MATERIALS Vemier Temperature Probe PROCEDURE 1 Obtain and wear goggles 2 Connect a Temperature Probe to Channel 1 of the Vernier computer interface 3 Start the Logger Pro program on your computer Allow Logger Pro to open the default program It should list columns for collecting time and temperature 4 Prepare a hot water bath by heating a halffilled 400 mL beaker on a hot plate 5 Mass 2 grams of potassium nitrate and record the actual mass in your lab book 6 Transfer the salt into your 10 mL graduated cylinder 7 Add distilled water to 2 mL in your graduated cylinder and observe if the dissolution of KNO3 is an exothermic or endothermic process feel the outside of the cylinder 8 In your hot water bath while stirring gently with the temperature probe heat the cylinder containing the salt and water mixture Do not leave it in the water any longer than necessary to get the salt into solution 9 Remove your sample from the hot water bath and record the total volume of the solution only do this after all the salt has gone into solution and do not forget to remove the temperature probe from the solution when measuring the volume 10 While monitoring the temperature allow the sample to cool while slowly stirring Record the temperature at the point when the first crystals appear The solution will appear to be snowing The solution is considered to be at equilibrium when the first crystals begin to appear At higher concentrations the process happens quite quickly 11 Remove the probe and add between 05 and 1 mL of distilled water Be careful not to lose solid as you remove the thermometer probe from the cylinder to add the water 12 Repeat steps 8 11 four more times until a total of 5 sets of data at different concentrations have been recorded 148 13 Disposal Place the KNO3 solution into the waste jar labeled KNO3 Waste For easier removal of the KNO3 reheat the solution until the entire solid has re dissolved and quickly pour it into the Waste jar In this case the Water can be evaporated and the potassium nitrate reused PROCESSING THE DATA For each set of data calculate the molar concentration of KNO3 and use that value to deduce both Kaq and N03 quotam concentrations Use equation 1 to calculate the Ksp for each data set Substitute Ksp into equation 3 to calculate AG for each data set Determine the natural logarithm ln of the Ksp at each temperature After converting all temperatures into Kelvin calculate the reciprocal of each temperature 1T Using a spreadsheet program Logger Pro has this capability or you can use Excel construct a graph with the y axis being lnKsp and the X axis being 1T Determine the best linear fit of the data using linear regression and record the slope and intercept in your notebook From your graph determine the values for AH and AS Equation 4 shows that AH of the reaction can be determined using the slope of the straight line from the graph while AS of the reaction can be determined from the yintercept 149 Thermodynamics of the Solubility of Potassium Nitrate Lab Report Your report for this lab should include the following sections Abstract Report the Ksp T Values and the calculated entropy enthalpy and free energy changes Introduction Give the equations and a brief description of how they are used to obtain the thermodynamic properties in this experiment Data Include a copy of the graph with the data indicating the slope and y intercept Results Calculate the Values of solubility Ksp and AG for each data set From the graphical analysis of your data calculate Values for AH and AS Discussion Use the data in Appendix II of your test book to calculate the AH and AS for the dissolution of KNO3 Calculate your percent error with respect to each result AH and AS Discuss the experiment and any possible sources of error Answer the following question as part of your discussion 1 Compare the signs of your experimental thermodynamic Values with your expectations from the prelab a Did you expect the dissolving process to be spontaneous Do your data confirm your hypothesis b Was this process endothermic or exothermic Does this observation match your calculated enthalpy Values c Did you expect this process to result in an increase or decrease in disorder How does this compare with your calculated entropy Values 150 Prelab Redox Titration Analysis of a Commercial Bleach Part A Answer the following questions in your lab notebook be sure to show your work for any calculations 1 Define oxidation Define reduction OIL RIGLeo the lion goes Ger 2 Write balanced oxidation and reduction half reactions for the following redox reaction equations For each halfreaction identify which substance is oxidized and reduced Eqn 1 Mn04 S203 2 9 S406 2 Mn 2 Eqn 2 Mn04 C204 2 9 Mn02 C02 3 Use the equations given below in the introduction of this experiment to determine the mole ratio of thiosulfate S2032 to hypochlorite ClO 4 500 mL of commercial bleach was diluted to 1000 mL 250 mL of the diluted sample was titrated with 456 mL of 0100 M S2032 What is the concentration of hypochlorite in the original bleach solution Assume the density of the commercial bleach is 108 gmL Calculate the average percent by mass of NaClO in the commercial bleach Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 151 Lab Redox Titration Analysis of a commercial Bleach Solution Many commercially available consumer products such as bleaches and hair coloring agents contain oxidizing agents The most common oxidizing agent in bleach is sodium hypochlorite NaClO sometimes written NaOCl Commercial bleach is made by bubbling chlorine gas through a sodium hydroxide solution Some of the chlorine is oxidized to the hypochlorite ion C10 and some is reduced to the chloride ion C1quot The solution remains strongly basic The balanced chemical equation for the process is Cl2g 2 OH39aq gt ClO39aq Cl39aq H2Ol The amount of hypochlorite ion present in a solution of bleach can be determined by an oxidation reduction titration One of the best methods is the iodine thiosulfate titration procedure The iodide ion 139 is easily oxidized by almost any oxidizing agent In acidic solution hypochlorite ions oxidize iodide ions to form iodine 12 The iodine that forms is then titrated with a standard known concentration solution of sodium thiosulfate The analysis takes place in a series of steps 1 An acidified iodide ion solution is added to hypochlorite ion solution and the iodide is oxidized to iodine hypochlorite is quantitatively converted to iodine 2Haq ClO39aq 2I39aq gt Cl39aq I2aq H2Ol 2 Iodine is only slightly soluble in water but it dissolves very well in an aqueous solution of iodide ion in which it forms a complex ion called the triiodide ion Triiodide is a combination of a neutral I2 molecule and an 139 ion The triiodide ion is yellow in dilute solution and dark red brown when concentrated I2aq I39aq gt I3 aq 3 The triiodide is titrated with a standard solution of thiosulfate ions that reduce the iodine back to iodide ions I339aq 2S2O3239aq gt 3 I39aq S4O6239aq During this last reaction the red brown color of the triiodide ion fades to yellow and then the color disappears when you reform the clear iodide ion solution It is possible to use the disappearance of the color of the triiodide ion as the method of determining the end point but this is not a very sensitive procedure it s hard to tell when the yellow color just disappears Addition of starch to a solution that contains iodine or triiodide ion forms a reversible blue complex The disappearance of this blue colored complex is a much more sensitive method of determining the end point However if the starch is added to a solution that contains a high concentration of iodine the complex that fonns may not be reversible Therefore the starch is not added until shortly before the end point is reached light yellow solution Then the solution turns blue and you titrate until the blue color goes away The volume of thiosulfate used during the titration and its known concentration are converted to moles which are related to moles of hypochlorite through reactions 3 2 and 1 and the hypochlorite concentration is calculated 152 OBJECTIVE In this experiment you will determine the concentration of hypochlorite ion present in commercial bleach MATERIALS 50 mL buret PROCEDURE 1 Obtain about 100 mL of diluted commercial bleach Note that this bleach solution has been made by diluting 5 mL of commercial bleach into 100 mL meaning that the sample you are working with is 20 times more dilute than the commercial strength You will need to scale back up for the final answer to this experiment Weigh out approximately 1 g solid KI This is a large excess over what is needed Pipet 2500 mL of the dilute bleach into an Erlenmeyer ask Add the solid KI and about 25 mL distilled water Swirl to dissolve the K1 Working in a fume hood slowly and with swirling add approximately 2 mL of 3 M HCl The solution should be dark yellow to redbrown from the presence of the 13 complex ions Obtain 70 mL of the sodium thiosulfate solution and use a few mL to rinse your buret then fill it as usual Record the concentration of the thiosulfate in your lab notebook Bring the redbrown triiodide solution back to your bench and titrate the iodine with the standard 010 M sodium thiosulfate solution until the iodine color becomes light yellow Add one dropperfull of starch solution The blue color of the starch iodine complex should appear Continue to titrate until one drop of Na2S2O3 solution causes the blue color to disappear Repeat the titration beginning with step 2 two more times PROCESSING THE DATA 1 Use the equations given in the introduction to determine the mole ratio of sodium thiosulfate to sodium hypochlorite Using the volume of sodium thiosulfate needed for titration of 2500 mL of diluted bleach calculate the molarity of the diluted bleach hypochlorite ion Calculate the molarity of the hypochlorite ion in commercial bleach undiluted Assuming that the density of the commercial bleach is 108 gmL calculate the percent by mass of NaClO in the commercial bleach Calculate the average percent by mass of NaClO in commercial bleach from your three trials 153 6 Read the label of the commercial bleach to find the percent by mass NaClO that is reported Calculate the percent error of your Value assuming that the label Value is correct 154 Analysis of a Commercial Bleach Solution Lab Report Your lab report should include the following sections Abstract Be sure to provide a percent by mass of sodium hypochlorite in bleach and the percent error in your measurement Introduction Include the relevant reactions and information as to how the concentration of sodium hypochlorite is determined Data Include the recorded concentration of the thiosulfate standard used for the titration the labeled Value of the concentration of commercial bleach and any data tables necessary for the completion of the lab Results Tabulate your results for the calculated concentration of bleach its labeled Value and the percent error Attach your hand Written calculations to your report Discussion Discuss the experiment and any possible sources of error 155 Prelab Electrochemistry Voltaic Cells Part A Answer the following questions in your lab notebook be sure to show your work for any calculations Use the table of standard reduction potentials in your text or another approved reference to complete the following table An example is provided Electrodes Halfreactions Net Reaction E E ce Zn Zns gt zn2 2equot Zn 5 Cu2quot gt Cu s Zn2quot 076 v Cu Cu2 2e gt Cus 034 V Cu Pb 110V Pb Ag Pb Mg Pb Zn Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 156 Lab 6 Electrochemistry Voltaic Cells In electrochemistry a Voltaic cell is a specially prepared system in which an oxidation reduction reaction occurs spontaneously but pairs of reactants and products are physically separated so the reaction can be harnessed This spontaneous reaction produces an easily measured electrical potential Voltaic cells have a Variety of uses e g batteries In this experiment you will prepare a Variety of semi microscale Voltaic cells in a 24well test plate Fig 1 A Voltaic cell may be constructed by using two metal electrodes and solutions of their respective salts which also serve as the electrolytes for the cell with known molar concentrations The circuit is then completed by connecting the solutions in this case with a salt soaked piece of string In Parts I and II of this experiment you will use a Voltage Probe to measure the potential of a Voltaic cell with copper and lead electrodes You will then test two Voltaic cells that have an unknown metal electrode and through careful measurements of the cell potentials identify the unknown metal In Part III of the experiment you will measure the potential of a special type of Voltaic cell called a concentration cell You will then use the Nernst equation to calculate the theoretical Value for the E0611 of your concentration cell and compare this to your measured Value Figure 1 OBJECTIVES In this experiment you will o Prepare a Cu Pb Voltaic cell and measure its potential 0 Test two Voltaic cells that use an unknown metal electrode and identify the metal o Prepare a copper concentration cell and measure its potentialUse the Nernst equation to calculate the E0611 of your copper concentration cell 157 MATERIALS Voltage Probe 24well test plate String Cu and Pb electrodes two unknown electrodes labeled X and Y steel wool PROCEDURE Part I Determine the E for a CuPb Voltaic Cell 1 2 Obtain and wear goggles Use a 24well test plate to construct your voltaic cells Drop small amounts of 010 M CuNO32 and 010 M PbNO32 solution into two neighboring wells in the test plate CAUTION Handle these solutions with care If a spill occurs ask your instructor how to clean up safely Obtain one Cu and one Pb metal strip to act as electrodes Polish each strip with steel wool Place the Cu strip in the well of CuNO32 solution and place the Pb strip in the well of PbNO3 solution These are the half cells of your CuPb voltaic cell Make a salt bridge by soaking a short length of string in a beaker than contains a small amount of 1 M KNO3 solution Connect the Cu and Pb half cells with the string Connect a Voltage Probe to Channel 1 of the Vernier computer interface Connect the interface to the computer with the proper cable Start the Logger Pro program on your computer Open the file 20 Electrochemistry from the Advanced Chemistry with Vernier folder Measure the potential of the CuPb voltaic cell as described below Complete the steps quickly to get the best data a Click to start data collection Connect the leads from the Voltage Probe to the Cu and Pb electrodes to get a positive potential reading Click KEEP immediately after making the connection with the Voltage Probe Remove both electrodes from the solutions Clean and polish each electrode Put the Cu and Pb electrodes back in the correct solutions to recomlete the voltaic cell Connect the Voltage Probe to the electrodes as before Click KEEP immediately after making the connection with the Voltage Probe Remove the electrodes Clean and polish each electrode again Reconnect the voltaic cell a third and final time Click p KEEP immediately after making the connection with the Voltage Probe Click E to end the data collection Click the Statistics button Record the mean in your data table as the average potential Close the statistics box on the graph screen by clicking the X in the corner of the box 158 Part II Determine the E for Voltaic Cells Using Pb Cu and an Unknown Metal Choose one unknown eitherX or Y and mentally substitute Yinto the directions that follow if you chose Y 8 Obtain a small amount of the unknown electrolyte solution labeled O 10 M X and the corresponding metal strip X 9 Transfer a small amount of 010 M X solution to a well adjacent to the 010 M PbNO32 solution in the test plate 10 Make a new salt bridge by soaking a short length of string in the beaker of 1 M KNO3 solution Connect the X and Pb half cells with the string 11 a b Fquot Measure the potential of the XPb voltaic cell Complete these steps quickly Click to start data collection Connect the leads from the Voltage Probe to the X and Pb electrodes to get a positive potential reading Click immediately after making the connection with the Voltage Probe Remove both electrodes from the solutions Clean and polish each electrode Set up the voltaic cell again Connect the Voltage Probe as before Click immediately after making the connection with the Voltage Probe Remove the electrodes Clean and polish each electrode again Test the voltaic cell a third time Click ii KEEP immediately after making the connection with the Voltage Probe Click to end data collection Click the Statistics button Record the mean as the average potential and then close the statistics box on the graph screen by clicking the X in the comer of the box 12 Repeat Steps 811 using the copper halfcell with the unknown metal halfcell don t forget to make a new salt bridge Part III Prepare and Test a Concentration Cell 13 Construct and test a copper concentration cell a Using your 010 M CuNO32 prepare 20 mL of 0050 M CuNO32 solution by mixing 10 mL of 010 M CuNO32 solution with 10 mL of distilled water b Create a concentration cell in two wells of the 24well test plate by adding 0050 M CuNO32 solution to one well and 10 M CuNO32 solution to a neighboring well Use Cu metal electrodes in both wells Use a KNO3soaked string as the salt bridge as in Parts I and II c Click to start data collection d Test and record the potential of the concentration cell in the same manner that you tested the voltaic cells in Parts I and II ie do three trials and polish the electrode each time 159 Voltaic Cells Lab Report DATA TABLE 1 Determine the E for a CuPb voltaic cell Voltaic Cell Cu Pb Measured Potential V Trial 1 Trial 2 Trial 3 Mean 1 Compare the average cell potential for your CuPb cell with the Eoceu that you calculated in the pre lab exercise Comment on your cell potential Why might it be different from the literature Value You may Wish to Write the Nernst equation for the cell you created DATA TABLE 2 Determination of the E for unknown metal MX using Pb Voltaic Cell Pb Mx Measured Potential V Metal connected to Lead Metal connected to Lead Trial 1 Trial 2 Trial 3 Mean DATA TABLE 3 Determination of the E for unknown metal MX using Cu Voltaic Cell Cu MX Measured Potential V Metal connected to Lead Metal connected to Lead Trial 1 Trial 2 Trial 3 Mean 160 2 The unknown metals X and Y were either magnesium silver or zinc Use the literature Value look it up in your text or on the Web if the latter cite your source for the reduction potential of Pb and Cu and the measured cell potentials for the unknowns to identify X or Y The cell potential for MK can be determined using the following equation EMX Eceu E kn0Wn Where E kn0Wn is the literature Value for copper or lead DATA TABLE 4 Determination of the E for a copper concentration cell Voltaic Cell Measured Potential 005 M 10 M V Trial 1 Trial 2 Trial 3 Mean 3 Part III Use the Nernst equation to calculate the theoretical Value of E for the copper concentration cell and compare this Value with the cell potential that you measured 161 Prelab Synthesis of Acetaminophen Part A Answer the following questions in your lab notebook be sure to show your work for any calculations 1 Write the balanced chemical equation for the synthesis of acetaminophen from p aminophenol and acetic anhydride 2 Starting with 15 grams of paminophenol and an excess of acetic anhydride calculate the theoretical yield of acetaminophen in grams It will be necessary to look up the molecular formulas of both the limiting reactant and the product online Part B Prepare your notebook for the lab This includes stating the purpose of the experiment summarizing the procedure in a bulleted list format be sure to include space for observations and preparing any tables necessary for data collection At the start of your lab remove the copies of the pages where you completed the above work from your lab notebook and turn them into your TA 162 Synthesis of Acetaminophen Acetaminophen N 4 hydroXyphenylethanamide is a relatively simple organic compound that is a common over thecounter pain reliever and fever reducer Figure 1 Acetaminophen is generally considered a safe medication in the US although overdoses are relatively common and can cause fatal liver damage Organic compounds such as Acetaminophen are generally classified by their functional groups Acetaminophen consists of a benzene ring core that has a hydroxyl functional group OH attached to one side and an amide functional group Figure 2 on the opposite side referred to as the para position see Figure 1 10 0 Figure 1 The molecular structure of Acetaminophen The apex and juncture of each line represents a carbon atom potentially bound with various numbers of hydrogen to give each carbon a total of four bonds I Figure 2 The structure of an amide R R and R represent various other organic groups or carbon chains Find the amide group in Figure 1 The synthesis of acetaminophen is performed by reacting paminophenol with acetic anhydride A byproduct of this reaction is acetic acid as shown Figure 3 In this lab you will start with 15 grams of paminophenol to synthesize the crude acetaminophen product The crude product will be purified through a common technique called recrystallization Recrystallization involves dissolving a crude product in a minimal amount of hot solvent Once the solution cools a more pure form of your product will recrystallize precipitate out of solution One way of assessing the purity of a substance is to determine its melting point or melting point range The range is reported from the temperature at which the first drop of liquid appears to the temperature at which the sample is completely melted and only a clear liquid is present A pure solid generally melts over a narrow range of temperatures and the melting temperature range will match the substance s melting point from the literature An impure substance may melt over a wide range of temperatures usually at a temperature lower than that of the pure substance The greater the amount of impurities the greater the decrease in the apparent melting point Here you will measure the melting point of your purified product and compare it to the accepted melting point range of acetaminophen 1695 171 C 163 OH OH O 0 O O 11 A 1 H2N 0 M 0H paminOphen01 acetic anhydride OI4hydroxyphenylethanamide acetic acid Figure 3 Synthesis reaction of acetaminophen Safety Precautions 0 Wear safety glasses or goggles at all times in the laboratory 0 Acetic anhydride is corrosive and its vapor is irritating to the respiratory system Avoid skin contact and inhalation of the vapors In the event of skin contact rinse well with cold water If the vapors are inhaled move to an area where fresh air is available 0 Phosphoric acid is corrosive Avoid skin contact In the event of skin contact rinse well with cold water 0 p aminophenol is harmful by inhalation and by contact with the skin In the event of skin contact rinse well with cold water If the vapors are inhaled move to an area where fresh air is available 0 NOTE Don39t use your acetaminophen for a headache Its purity is not assured Procedures Synthesis of Acetaminophen 1 Weigh out 15 g of salicylic acid Place it in a 125 mL Erlenmeyer ask 2 Add 25 mL of water and 25 drops of 85 H3PO4 3 Gently heat to dissolve 4 Move to the hood once all of the solid has dissolved Working in the hood while swirling the ask add 2 mL of the acetic anhydride Allow this to react for 5 minutes 5 Place the mixture on ice and allow the solid to crystalize for 20 minutes If crystallization does not occur it may be necessary to scratch the side of the ask with a stir rod or add a small seed crystal of pure acetaminophen ask your TA 6 Collect the crystals by vacuum filtration using a Buchner funnel 7 Wash the crystals twice on the filter with 5 mL portions of ice cold water Recrystallization of the Acetaminophen 1 Place the crude acetaminophen crystals in a clean 150 mL beaker Add 20 mL of distilled water and heat on a hot plate until all of the solid has dissolved If the solution starts to boil and undissolved solid still remains add more water a few mL at a time until the solid dissolves 2 Remove the beaker from the heat and allow the solution to cool When crystals begin to appear put the solution on ice cooling for 20 minutes If crystallization does not occur it may be necessary to scratch the side of the ask with a stir rod 3 Collect the crystals on a pre weighed piece of filter paper using the Buchner funnel Wash the product on the filter paper with two 5 mL portions of ice cold water Allow the purified product to dry for 10 minutes under vacuum 164 4 Weigh the product It may be necessary to dry for a few more minutes if it still appears Wet Record the final dry Weight in your lab notebook for the calculation of yield Determine the Melting Point of the Acetaminophen Sample 1 Fill a capillary melting point tube to a depth of 02 cm with the recrystallized product as demonstrated by your TA 2 Place the capillary tube in the melting point apparatus Determine its melting point range Your TA will demonstrate the use of this apparatus Pure acetaminophen melts at 1695171 C 3 The acetaminophen sample should be labeled with your name the mass of the acetaminophen your percent yield and the melting point range 165 Synthesis of acetaminophen Worksheet no lab report g Mass of paminophenol g Theoretical yield of acetaminophen show your Work below g Mass of pure product Percent yield show your Work below C Accepted melting range of acetaminophen C Measured melting point of acetaminophen What does the comparison of your measured melting point to the known melting point tell you about the purity of your product During the crystallization of acetaminophen What was the purpose of cooling the sample in an ice bath Why should you use a minimum amount of hot solvent to dissolve the raw product during the recrystallization step 166