CHEM 227 General Chem Lab Manual
CHEM 227 General Chem Lab Manual
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Chemistry 227 228 and 229 General Chemistry Labs 20132014 Academic Year Lab Coordinators Dr Eric Sheagley sheagleypdxedu Dr Dean Atkinson atkinsdbpdXedu 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 Lab Scientific Measurements Precision and Accuracy 23 PreLab Who has the same solid that I have 28 PreLab How much sugar is in a can of coke 34 PreLab A Cycle of Copper Reactions 41 Prelab Which Alkali Metal Carbonate 47 Prelab Using Conductivity to Find an Equivalence Point 53 Prelab Atomic Emission Spectra 58 PreLab Determining the Concentration of a Solution Beer s Law 67 Electron Density Prelab 73 CH228 LABS 78 PreLab Enthalpy of Neutralization of Phosphoric Acid 78 PreLab Hess s Law 83 Deriving the Gas Laws Using Computer Simulations 87 PreLab Decomposition of Hydrogen Peroxide 92 PreLab Vapor Pressure and Heat of Vaporization 97 PreLab Using FreezingPoint Depression to Find Molecular Weight 103 PreLab The Rate and Order of a Chemical Reaction 109 PreLab Chemical Equilibrium Finding a Constant Kc 114 Le Chatelier s Principle in a Cobalt Complex 121 CH229 LABS 124 Prelab Acid Rain 124 Prelab Acid Dissociation Constant Ka 129 Prelab Titration of a Diprotic Acid 133 Identifying an Unknown 133 Prelab Buffers 141 Prelab Determination of the Ksp of Calcium Hydroxide 146 Prelab Thermodynamics of the Solubility 152 of Potassium Nitrate 152 Prelab Redox Titration Analysis of a Commercial Bleach 157 Prelab Synthesis of Acetaminophen 161 Prelab Electrochemistry Voltaic Cells 166 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 6 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 copy 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 docllted 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 it 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 slltill 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 0 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 0 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 0 Headphones may not be worn in lab Safety Procedures 0 Know location of safety equipment re extinguisher re blanket first aid kit safety shower eyewash fountain and all exits 0 In case of fire or accident call the instructor at once 0 Small fires may be extinguished by wet towels 0 If a person s clothing catches fire roll the person in the fire blanket to extinguish the ames 0 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 0 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 0 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 0 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 0 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 0 Leave all equipment and work areas as you would wish to find them 0 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 0 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 clean up 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 0 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 0 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 0 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 0 Do not take the reagents to your bench 0 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 0 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 0 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 0 Do not put any excess reagent back in the reagent jar Treat it as waste and dispose of it properly 0 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 0 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 0 Any chemicals that come in contact with your skin should be immediately washed with soap and copious amounts of water Laboratory Procedures 0 Never pipet any liquid directly by mouth Use a rubber bulb to draw liquid into the pipet Never weigh hot chemicals or equipment 0 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 0 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 0 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 0 Use the fume hood for all procedures that involve poisonous or objectionable gases or vapors 0 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 0 Use black ink finetipped ball point pen this will photocopy clearly 0 At the front of the notebook leave a few pages for a Table of Contents 0 Each lab should have a brief introduction and description of procedure 0 Generally use only the right hand page for most text 0 Use facing left page for working graphs manual calculation and working notes 0 Prepare data tables in advance with columns for calculated results and notes 0 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 wellthoughtout 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 T s 5h pp t w pEs t P7p pCq 0 r gq q I pEs I Ct Qq q P p Es pp p r m r r q I 3921 rag1 qp Ns t r n 8r 03h Ss Wn 0 s 1q I p 0 2 P0t 4 M wrz s n i F5 0 p pn SW 0 6 Pt 5 do o 5 q o Jd E r Pf r Ir Vr p q ECBR 109 q M39 3i1 xp3914391quot Cquot l I l o Zp p U Zn I I e Wquot o 2U 2 KJ l Lg A 2 we eager 0On L 3 3 D0 m mw krmfu rm we ueL H n l F quot4quotquot 393 U Ger k g l P g k m h m lg f mg wLp 1sCM L4 e e e e f ck Peampg3r 3 spa quotr t Ze g Ze 0 h gt quot39quotS pJe f p j j 3 h I 5 P4i quotuc39 39r39 EI I 39 xJl 39 a u d c Pae I quot3p lJ39iriI relrcaH5 11 1F391 L rNl w l 3913935rzuELr5 qf S gleJ1r I I quot uf 39 561d Dquot eu 3939l39Ir 5J3 39F 51 E per l E V qe V 1 r39 3 3 39ILg W 9 Kc J A pL7 Ls a W V A p T Vd 1 r Z N A d 39 0 JIC3 b b d 3e5V5 T P Z e d p H4 0ib F oc b S d Wa c NOTE IMSEHT PEHIDBIE TFLBLE UNDER C3P Y SHEET BEFURE VWR TfNG 0 THE HM3939E39IHMcHiElL STUDEl 1IT IIJLE M T39EE K i l quot 39 V 1 quot539 f x k X L4 5 PI 67L p fsa 5 1 P T1L Q 4 5 p5 3 3 T P A 8 p p NCITE rNSEHT PERIODIC MELE UNDER COPEquot SHEET BEHJHE 39W39R 39TNG THE HM39DEHWInHElL STUIJEHT LEE H TEiBfl H 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 10 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 Stvle 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 5O 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 0 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 standalone 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 0 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 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 Graphs When graphical presentation of data is necessary please prepare graphs using the following guidelines 0 Number figures 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 the graph that indicates what is being plotted on the yaxis 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 08 E C N 06 Fl LO 3 04 G E 5 02 398 0 quot7 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 t 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 drnv equation 1 which was explained in the introduction d 394 g355 mL111gmL 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 pipette was used to precisely deliver 10 mL of water The mass of the 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 how much liquid is actually delivered a pipette can be calibrated Calibration refers to the comparison of the actual amount of liquid delivered by the pipette to the standard value of the pipette In this experiment a 10 mL pipette was calibrated by delivering 10 mL of water to a beaker that was then weighed By using the weight of the water transferred taking into consideration the time it took to weigh the samples and factoring in the amount of water that evaporated during the time of weighing the actual amount of water transferred with the pipette was found Because delivered volume was calculated another measurable quantity was used to verify the volume delivered by the pipette In this case the relationship between mass and volume density was used Mass is an easily measurable quantity that can be determined with a high degree of accuracy due to the availability of electronic balances and can be converted to volume by the use of density 9 mass 9 density volume mL The density of water at a variety of temperatures is readily available and was 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 18 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 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 Avera e mass 0 water Volume mL g f g g Known density of water at temperature 996 g Volume 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 water evaporated g Mass evaporated g Rate of evaporation lt 60 Seconds gt Time of evaporation seconds 00016g 60 s Mass evaporated g 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 Mass transferred g Mass of water weighed g Mass of water evaporated g Mass transferred g 99769 g 00024 g 99793 g Calculation of Percent Error theoretical value experimental value Percent Error I 100 theoretical value 1000 998 moo 100 02 Percent Error 19 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 as 994998 g on the pan balance and 9973599769 g on the analytical balance tables 1 and 2 respectively The average mass of water delivered was then converted to volume using the density of water for the pan balance and the analytical balance measurements 996 g to 998 mL of water and 9978 g to 9998 mL of water respectively In the case of the analytical balance the rate of evaporation of water which is a systematic error was taken into consideration for each trial before the average was taken 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 Trial 1 of the analytical balance had the most evaporation due to a longer time passing between delivering water and weighing the water while trial 3 of the analytical balance had the least amount of water evaporate table 4 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 2710395 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 If enough water were not drawn into the pipette to meet the 10 mL line the amount of water delivered to the balance would decrease which would in turn decrease the volume of water found to be delivered by the pipette Another error that could have occurred though unlikely to result in introducing high amounts of error would be having used a dirty pipette While most water was delivered to the beaker from the pipette small amounts of impurities on the inside of the pipette could lead to retention of some water inside the pipette not to be delivered to the beaker This would cause a decrease in the weight of the water in the beaker and a decrease in the volume calculated to have been delivered by the pipette 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 information to the reader 21 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 Lab Scienti c Measurements Precision and Accuracy Background Accuracy is a measure of how close a measurement is to the correct value For example the accepted density of zinc is 714 gmL One student may experimentally determine it to be 727 gmL and another may determine it to be 720 gmL Because the value 720 gmL is closer to the accepted value it is considered to be a more accurate measurement You will find that in some cases there is no accepted value to compare a result to In those cases accuracy cannot be measured Precision is a measure of the reproducibility of a measurement how close a set of measurements are to one another If in a series of trials you measured the density of zinc to be 726 gmL 728 gmL and 725 gmL your values are quite close to each other This suggests that your precision is good Precision also deals with how subtle of a change that can be measured For instance think about measuring your weight on a bathroom scale as compared to a roadside scale a scale the measures the weight of large trucks On a bathroom scale your mass might read 1556 pounds but on the roadside scale your mass may be given as 140 pounds The bathroom scale has the ability to measure relatively small masses to the nearest 01 pounds while the roadside scale has the ability to measure large masses to the nearest 20 pounds Measurements made of relatively small masses will be more precise with the bathroom scale reproducibly you will be able to measure a mass to the nearest 01 pounds The limited precision of the roadside scale would create a level of uncertainty in measuring small masses This uncertainty will restrict the conclusions that can be made from the measurement so a roadside scale would not be the instrument of choice for measuring a person s weight Scientists represent this precision in the equipment by putting writing i error after the measurement For example the first scale s measurement would be written as 1556 i 01 lb This shows that there is an uncertainty in the last digit of the measurement The roadside scale measurement would be written as 140 i 20 lb In this class we will use significant figures to indicate the level of precision with which a measurement has been made instead of writing the error using the i error notation The greater the level of precision the greater the number of digits in that measurement that are significant Signficant Figures For instance 155 6 lb has four significant figures meaning that all of the digits in the value are known with a relatively high degree of certainty The value 140 lb on the other hand has only 2 significant figures the one and the four Because of the scale used above we can only have a relatively high degree of certainty of the value to the tens place The more significant figures that are reported the more precise our measurement Just be sure not to report more significant digits than the instrument used for measuring will allow The manipulation of measurements with a know precision number of significant figures is included in your text book In general when making a measurement it is customary to include all the known values of a measurement plus one digit that is estimated Assuming that the bathroom scale is not digital suppose that when weighing yourself in the example above the needle points somewhere between 155 and 156 pounds You know your weight greater than 155 pounds but less than 156 pounds To arrive at the 155 6 pounds reported above you had to 23 estimate the 06 pounds the estimated value in the measurement In general estimate the value to one decimal place more than the level of graduation In the example above the graduation is every 1 lb Therefore the measurement is reported to the 01 lb Every measurement you make in this lab must include the proper number of significant figures To determine this one only needs to look at the graduations on the instrument used to make a measurement You will be limited to 1110 of the smallest known graduation Procedures You will be graded on the number of digits used the presence of units on your measurements and explanations Use a pen to record all measurements 1 Measurements using a graduated cylinder Your TA will have a large graduated cylinder filled with some amount of a liquid on display Without consulting anyone in your class and without sharing your value determine the volume of the liquid and record the value here Before continuing report your value to your TA Your TA will lead a quick discussion about your results before allowing you to continue When measuring volume using a graduated cylinder one always records the volume as the level at the TOP BOTTOM of the meniscus Circle the best I39CSpO1 1SC Take out a 25ml graduated cylinder Check to see whether your graduated cylinder is calibrated to every 1 mL or every 05 mL The 25mL cylinder is calibrated to every mL Since the graduations are every 05 mL you should try to report your volume to two decimal places Use the following diagram to estimate the second decimal place 24 With certainty you can see the liquid is slightly above 125 mL The bottom of the meniscus sits just above 125 mL Let s approximate that the liquid is 410 04 of the distance between the graduations 125 and 130 mL Since each graduation is 05 mL 04 X 05 mL 02 mL Add this to your certain digits 125 mL 02 mL 127 mL 127 mL should be recorded Place about 1020 ml of water into your 25mL cylinder and make a sketch of it below with at least two labeled graduations as in the previous drawing Be as accurate in your drawing as possible Report your measurement here Don t forget units 25 2 Measuring lengths Now find a small plastic ruler and measure your pencil Always use the smallest graduations on your instrument when taking a measurement Length cm Length mm Obtain a meter stick and record the length in meters if the meter stick is graduated to the mm be sure to add the appropriate number of digits in your response Length m Does your measurement have a different precision when you use a meter stick Explain 3 Measuring mass Obtain a coin Use the balance to measure the mass of the weight With a digital balance always record every digit the balance displays Record your units Mass Based on your observations how many decimal places does this balance report Is there a difference in reporting 1201 g versus 12010 g Explain how the meaning of the measurement changes 4 Comparison of a beaker and a graduated cylinder Take out a 100 or 150 ml beaker and examine its graduations Complete the following statements The beaker is graduated every mL The measurement should be recorded to every mL Add approximately 25 ml of water to your beaker using the graduations on the beaker Measure the volume of this water using the beaker and record it below mL using a beaker 26 How many significant figures can you report based on the graduations sig figs circle the digit above that was estimated Now take the contents of your beaker the 25 mL of water and pour it into your 100 or 150 ml graduated cylinder Thinking about what you leamed earlier looking at the graduations record a measurement for the volume of water mLusing a 100 or 150 ml graduated cylinder Which is more precise the volume measurement using the beaker or the graduated cylinder Given a choice which glassware would you use to measure volumes more precisely Explain Suppose a l0 mL graduated cylinder has an uncertainty of about i 001 mL If it is filled with water to the mark l0 mL the volume should be reported as include the necessary decimal places mL Provide a measurement for each of the following pieces of glassware filled to the level indicated by the arrow Use the correct number of significant figures in your answer Always use units Remember to estimate one digit more than the level of graduation mL ruL Ill mL 11 27 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 intemet 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 28 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 leaming 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 side by side 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 bum 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 pattems 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 a result 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 29 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 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 peasized 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 30 5 Determine if the aqueous solution is acidic basic or neutral a b 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 Record your observations 6 Determine the reactivity of your unknown a 6 f 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 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 Add 5 drops of 01 M leadII nitrate PbNO32 to the second well 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 produced Note aqueous solutions of ammonia are basic and the pH paper would tum blue in the presence of ammonia gas Record your observations such as turned 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 31 10 Run some confirmatory tests to Verify that you have the same compound as the other groups Choose two tests that makesyour unknown unique from the other substances and do a sidebyside comparison to Verify your conclusion 11 Clean up 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 DATA 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 unknownsPossible 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 32 Who has the same solid that I have Lab Report Your report for this lab should include the following sections Abstract Your firstdraft 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 the complete set of class data 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 Write a sentence or two or a table summarizing the matching and non matching characteristics to indicate which other solid matched your groups unknown specifically list which results were similar and which results were different I Summarize the results obtained for your unknown and the matching unknowns in the confirmatory tests 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 Describe howwhy you chose the confirmatory tests and the corresponding results 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 33 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 34 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 7 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 35 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 x axis 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 60 50 e 40 e 32 Q 30 e E 20 e 10 e O l l l l l 0 10 20 30 40 50 60 Volume ml Figure 1 The relationship between Mass and Volume for Water 60 50 i A 40 V 30 V 20 1o 0 Mass 9 0 1 0 2 0 3390 4 0 5390 60 Volume ml Figure 2 The relationship between Mass and Volume for Water Varying Volumes of water were massed A linear relationship exists between mass and Volume A best fit line was calculated using Microsoft Excel yielding the following equation y 100l5x 0009 Graphing Using Microsoft Excel An excellent tutorial on graphing with MSExcel can be found at the following website httpwwwncsuedulabwriteresgtgtmenuhtml 36 This is a list of the basic steps necessary to graph data and do a linear regression the generation of a bestfit 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 37 PROCEDURE Calibration Curve You will make one sample without sugar and ve 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 For the sample without sugar 1 Weigh the empty ask and record the mass 2 Add water carefully to the fill line 3 Weigh the 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 To accurately know the mass of sugar used in each of your five standard solutions follow these steps 1 Weigh the empty ask 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 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 or cover the top of the ask with Parafilm 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 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 Be sure that you add an appropriate caption to the table See the report guidelines section of the manual for a discussion on this 38 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 Pure H20 00 Flask 1 Flask 2 Flask 3 Flask 4 Flask 5 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 Alternatively 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 39 Sugar in Coke Lab Report Your report for this lab should include the following sections Abstract Your firstdraft abstract for this lab should be Written as part of a postlab discussion led by your TA Introduction Begin with a statement of the purpose of the experiment Provide any relevant background and key concepts Explain the calibration curve Include useful equations and relationships density 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 40 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 41 A Cycle of Copper Reactions GOALS 1 Cycle solid copper through a series of chemical forms via aqueousphase 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 CuNO3 L CuOH HNO3 Cu heat Zn Cl H SO C1130 4 2 CuO 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 acidbase reactions and oxidation reduction or redox reactions in addition the text may have discussed gasforming reactions In precipitation reactions soluble cations and anions combine to form an insoluble compound that leaves the solution as a solid precipitate In acidbase 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 abovedescribed types of aqueous reactions Reaction 1 The first reaction proceeds according to the following balanced chemical equation 4 HNO3 aq C11 S C11NO32 aq 2 H20 1 2 N02 g 42 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 oxidationreduction 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 acidbase 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 oxidationreduction 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 43 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 H2SO4 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 44 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 not blue the solution can be murky and grey 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 preweighed 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 copper 100 REFERENCE 1 Condike GF J Chem Ed 1975 52 615 45 Copper Cycle Lab Report Your report for this lab 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 percent recovery Were Introduction 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 Data Include each balanced chemical reaction and your observations for each reaction Include the initial and final mass of copper Results Report which of the observations that suggested that a chemical reaction took place heat precipitate color change formation of a gas and relate those observations to the components of the balanced chemical reaction the black solid Was 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 46 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 2 What is the law of conservation of mass How many moles of CO2 can be produced by the complete reaction of 153 g of lithium carbonate with excess hydrochloric acid balanced chemical reaction is given below Li2CO3s 2HClaq gt 2LiClaq H2Ol CO2g 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 How many moles of barium carbonate were produced How many moles of alkali metal carbonate were reacted What is the molar mass of the alkali metal carbonate Hint remember the units of molar mass are gmol 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 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 bu11eted 1ist 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 tum them into your TA 47 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 48 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 B aCl2 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 05 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 one piece of filter paper Be sure to record the mass in your notebook Filter the barium carbonate using the filter paper 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 combined dry solid and filter paper 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 MZCO3 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 KZCO3 What alkali metal carbonate is your sample most likely to be Where are the errors most likely to enter into the experiment 49 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 HCl gt CO2 2 MCI 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 05 g of your unknown in a pre weighed 250 mL beaker 2 Measure 400 mL of 1 M HCl in a pre weighed graduated cylinder Determine the actual mass of HCl added Pour the HCl 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 use the average from the three trials 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 carbonate is your sample 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 50 Flame test In this experiment you will make use of the fact that metal salts when heated in a ame emits light whose color is characteristic to the metal ion in the salt see table 1 The heat excites the electrons in the metal When the excited electrons relax to what is called their ground state the release a photon of light that has a characteristic energy or color if the photon is in the visible portion of the electromagnetic spectrum This property is used to identify unknown metals and even quantify their amounts present in a sample Table 1 Characteristic Colors for the Flame Test of Certain Group 1 Salts Group 1 Flame Element Color Li Crimson Na Golden Yellow K Violet Rb Blueviolet PROCEDURE 1 Dissolve a small amount the volume of a pea into about 5 mL of water 2 Dip the wire loop in concentrated hydrochloric acid 3 Heat the wire loop in the outer edge of the Bunsen Burner ame until no additional color is in the ame you may have to repeat steps 12 several times Dip the wire into your unknown solution Move the loop up and down in the outer edge of the ame and note the color of the ame Repeat steps 4 and 5 if necessary Folowing steps 1 and 2 clean the metal loop for the next user 1o u1gt Experiment adapted from Dudek E P J Chem Educ 1991 68948 51 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 three 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 Include ame test information Results Include a results table that summarizes your results from both methods be sure this includes all trials of each method Calculate the percent difference for the molar mass of the unknown metal carbonate with the molar mass of each of the three possibilities Li2CO3 Na2CO3 and KQCO3 Use this to assist in identifying your unknown metal carbonate Report your findings of the molar mass of your metal carbonate and the identity of the metal carbonate Attach hand written sample calculations to the back of your report Discuss if the ame test supports your identification Discussion Discuss the experiment and any possible sources of error Any suggested errors should be accompanied with a discussion as to how the error could have been responsible for the error seen ie a molar mass that was lower or higher than expected 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 52 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 of HZSO4 to the moles of BaOH2 originally present 4 If you add excess H2504 past the equivalence point what should happen to the conductivity of the solution in the flask 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 53 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 0 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 H2SO4 and barium hydroxide B aOH2 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 SO42aq A 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 Prelab 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 0 When the Conductivity Probe is placed in BaOH2 prior to the addition of H2SO4 As H2SO4 is slowly added producing BaSO4 and H20 0 When the moles of H2SO4 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 54 MATERIALS NEEDED magnetic stirrerstir plate if available Vernier Conductivity Probe 50 mL buret 250 mL beaker Measuring Volume Using a Buret l 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 H2504 is a strong acid and should be handled with care Obtain a 50 mL buret and 39 rinse the buret with a few mL of the H2SO4 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 For the filtration step your group will collaborate with the other groups at your lab bench Using a 100 mL graduated cylinder measure out the appropriate volume of the barium hydroxide solution for all the groups on your bench each group will need 250 mL of the BaOH2 solution Using a filter ask with two pieces of filter paper filter the BaOH2 solution Once filtered use a graduated cylinder to aliquot 25 mL to each group Transfer the solution to a clean 100 mL beaker Then add 15 mL of distilled water to the beaker this step just adds volume so that the probe can accurately measure the conductivity of the solution CAUTION Ba OH 2 is toxic Handle it with care Arrange the buret Conductivity Probe and beaker containing BaOH2 as shown in the picture above The Conductivity Probe should extend down into the BaOH2 solution to the bottom of the beaker 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 26a Conductivity to find Eqiv pt from the Chemistry with computers 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 55 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 d Next use 2drop increments 0l mL until the minimum conductivity has been reached at the equivalence point Read and enter the volume after each 2drop addition When you have passed the equivalence point continue using 2drop 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 8 When you have finished collecting data click Dispose of the beaker contents in the waste jar as directed by your TA 9 Print a copy of the table 10 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 HZSO4 2 Calculate moles of HZSO4 added at the equivalence point using the molarity M of the HZSO4 and its volume in L 3 Calculate the moles of BaOH2 at the equivalence point Use your answer in the previous step and the ratio of moles of BaOH2 and HZSO4 in the balanced chemical equation or the 22 ratio of moles of H to moles of OHquot 4 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 alternate 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 Drag your mouse cursor across the lineegion of data that precedes the minimum conductivity reading Click on the Linear Fit button 2 Drag your mouse cursor across the linear region of data that follows minimum conductivity reading Click on the Linear Fit button 3 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 56 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 57 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 58 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 59 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 m sec 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 60 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 1nzinitial Rydberg equation for Balmer series ninitia 2 A E he7 Links wavelength and energy be careful of the units lI11T1lX103991T1 61 39e 2 A s1mpi e1ransmissfan grat39i39ng spectmrscopa L 93 i an 39 39 Li EM Source Spec roscope Enha noe 39litKbT P C p L i QM Qcauhee Z K Trfarnamiaa ion 39Grquoti i r1g P 1 455 Linear pier Inch 6 VA 5IiEJ IImage r 4 13 quot G E391 S Figure 3 Maasu ng wmmknmhw ha transmission grating GMT R E dlst ama nquotramIalmaga Ia ini I 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 63 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 64 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 0 65 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 66 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 1M 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 67 Determining the Concentration of a Solution Beer s Law OBJECTIVES In this experiment you will 0 Prepare Allura Red standard solutions 0 Use a Colorimeter to measure the absorbance value of each standard solution 0 Find the relationship between absorbance and concentration of a solution 0 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 Beer39s Law Standard 3 Hbsnrhance Standard Concentration mulr L Figure I 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 68 NMJERMLSBEEDED Vernier Colorimeter one cuvette one 5 mL pipet pipet pump or pipet bulb one 50 mL volumetric ask PROCEDURE 1 Obtain and wear goggles 2 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 LoggerPr0 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 0 Handle cuvettes only by the top edge of the ribbed sides 0 All solutions should be free of bubbles 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 l00 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 2E6 You may need to click on the Autoscale button to rescale the graph as you go along 69 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 5 mL of the Gatorade to a clean volumetric ask Finish preparing your unknown by diluting the Gatorade to a total volume of 50 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 70 Allura Red has the following chemical structure 3 H 0 Ga T Li ll 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 modi ed from lab 11 Determining the Concentration of a Solution Beer s Law from Chemistry with Computers Third Edition Vernier Inc 71 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 72 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 Provide a brief explanation as to why atoms may have different values of electronegativity What causes a low electronegativity High Brie y describe the difference between a nonpolar covalent and polar covalent bond 73 Electron Density Lab In this atomiclevel simulation you will investigate how an atoms electronegativity value affects the types of bonds they produce Enter Phetc0l0rad0edu into the browser of your computer select Play with the Sims 9 Chemistry 9 Molecular Polarity Part A Select the two atom tab in the upper left hand corner of the simulator Turn on check all View options Investigate how the bond behaves when the atom39s electronegativity is changed 1 What does 39represent 2 What does the symbol 8 or 8 represent 3 Adjusting the electronegativities of atoms A and B determine if an atom with a high electronegativity can form a covalent bond Describe under what circumstances this can occur 4 How does changing the electronegativity of the atoms affect the bond polarity 5 Turn on check electron density in Surface options Describe how a molecule s polarity is related to it electron density How does the electron density around an atom with a 8 compare to the electron density around an atom with a 5 6 What scenarios can bring about a higher electron density around a particular atom 74 Electrostatic potential correlates with electron density Electrostatic potential is basically a measure of how a proton would react when brought to different regions of a molecule It provides a useful way to quickly predict polarity of a molecule or a region in a larger molecule Negative electrostatic potential colored in shades of red corresponds to an attraction of the proton Positive electrostatic potential colored in shades of blue corresponds to a repulsion of the proton 7 Describe how is electron density related to the electrostatic potential In the red shaded regions what is responsible for a potential attraction to a proton In the Blue shaded regions What would cause the repulsion 8 Explain how the direction of the arrow in the bond dipole symbol 4 relates to the electron density the partial charges and the electrostatic potential on a molecule 9 What happens to a polar molecule when the electric field is turned on Make sure to spin the molecule several times While making observations What if the molecule is nonpolar 75 Part B Select the three atoms tab in the upper left hand corner of the simulator Tum on check all view options Investigate how the bond behaves when the electronegativity of the individual atoms is changed In addition to changing the electonegativities orient the radial atoms relative to one another by dragging them with the mouse 1 Provide a summary as to how the bond dipoles and geometric orientation of the bonded atoms affect the molecular dipole 2 Provide a scenerio in which a molecule with two strong bond dipoles can have no molecular dipole at all Explain your answer with a drawing showing individual bond dipoles and the overall molecular dipole 3 Provide a scenerio in which a molecule may have a very large molecular dipole Explain your answer with a drawing showing individual bond dipoles and the overall molecular dipole Part C Describe properties between molecules explored more in CH222 One such is the solubility of one substance in another There is an adage that describes this ability quotLike dissolves likequot molecules with similar molecular dipoles will tend to interact favorably and mix For instance a polar molecule will mix well dissolve other polar molecules ethanol readily dissolves in water both are polar molecules and possess strong molecular dipoles Octane a major component of gasoline will not dissolve in water because it does not have a molecular dipole and is thus a nonpolar molecule Being able to predict the polarity of a molecule is extremely important since many properties of molecules depend on whether they are polar or nonpolar Predicting a molecule s polarity is a 76 multi step process that starts with drawing the Lewis structure Using VSEPR predict the molecule s molecular geometry Individual bond polarities are finally used to predict the molecular polarity 1 For the following molecules complete this stepbystep process Molecule Lewis Structure Molecular 3 d Geometry with Bond Polar or Geometry Polarities nonpolar Tetrahedral 1 1 7k CH3F i 0 d l overall diple HCH W C POLAR H H H H H N2 BF3 CH2F2 C is the central atom HCN C is the central atom Make a prediction and then check it in the Real Molecules section of the simulation 2 Using the molecular dipolespolarity of BF3 explain Why BF3 does not mix With H20 77
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