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Week 2 Notes

by: Hallie Hawkins

Week 2 Notes CHE105

Hallie Hawkins
Allen County Community College

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About this Document

These notes cover Chapter Two
Introduction to Chemistry
Dr. Saha
Class Notes
Chemistry, General Chemistry
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This 10 page Class Notes was uploaded by Hallie Hawkins on Sunday September 25, 2016. The Class Notes belongs to CHE105 at Allen County Community College taught by Dr. Saha in Fall 2016. Since its upload, it has received 7 views. For similar materials see Introduction to Chemistry in Chemistry at Allen County Community College.


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Date Created: 09/25/16
Chapter Two Basic Units and Symbols ● The English system was used primarily in the British empire ● The French organized a committee to devise a universal measuring system ● After about 10 years, the committee designed and agreed on the Metric system ● The metric system offers simplicity with a single base unit for each measurement  Original Metric Unit Definitions  ● A meter was defined as 1/10,000 of the distance from the North Pole to the  equator  ● A kilogram was equal to the mass of a cube of water measuring 0.1 m on each  side  ● A liter was set equal to the volume of one kilogram of water at 4 C Metric Prefixes ● The following table lists the common prefixes used in the metric system  Metric Prefixes, Continued ● For example, the prefix kilo­ increases a base unit by 1000: ○ 1 kilogram is 1000 grams ● The prefix milli­ decreases a base unit by a factor of 1000: ○ 1000 millimeters is 1 meter Metric Symbols ● The names of metric units are abbreviated using symbols. Use the prefix symbol  followed by the symbol for the base unit ○ Kilometer is abbreviated km ○ MIlligram is abbreviated mg ○ Microliter is abbreviated mL ○ Nanosecond is abbreviated ns Critical Thinking: The International System of Units (SI) ● An advantage of the metric system (i.e. International System of Units, SI) is that it is a decimal system  ● It uses prefixes to enlarge or reduce the basic units ● For example: ○ A kilometer is 1000 meters ○ A millimeter is 1/1000 of a meter Metric Conversion Factors ● A unit equation relates two quantities that are equal   ● For example ○ 1 kilometer= 1000 meters ○ 1 km= 1000 m  ● Also, we can write ○ 1 centimeter = 1/100 of a meter ○ 1 cm = 0.01 m Unit Factors ● A unit conversion factor, or unit factor, is a ratio of two equivalent quantities  ● For the unit equation 1 m = 100 cm, we can write two unit factors  Metric­Metric Conversions ● An effective method for solving problems in science is the unit analysis method ● It is also often called dimensional analysis or the factor­label method   ● There are three steps to solving problems using the unit analysis method  ○ Read the problem and determine the unit required in the answer ○ Analyze the problem and determine the given value that it is  related to the answer ○ Write one or more unit factors to convert the unit in the given  value to the unit in the answer  Applying the Unit Analysis Method Metric Equivalents  ● We can write unit equations for the conversion between different metric units ● The prefix kilo­ means 100 basic units, so 1 kilometer is 1000 meters  ● The unit equation is 1 km = 1000 m  ● Similarly, a millimeter is 1/1000 of a meter, so the unit equation is 1000 m = 1 m Metric Unit Factors ● Since 1000 m = 1 km, we can write the following unit factors for converting  between meters and kilometers: ○ ● Since 1000 m = 1 m, we can write the following unit factors: ○ Metric­Metric Conversion Problem  ● What is the mass of a 325­mg aspirin tablet? ○ Step 1: We want grams  ○ Step 2: We write down the given: 325­mg. ○ We apply a unit factor (1000 mg = 1 g) and round to three sig figs. Two Metric­Metric Conversions ● A hospital has 125 deciliters of blood plasma. What is the volume in millileters? ● Step 1: We want the answer in mL ● Step 2:  We have 125 dL ● Step 3: We need to first convert dL to L and then convert L to mL? Two Metric­Metric Conversions, Continued ● Apply both unit factors, and round the answer to three sig figs ● Notice that both dL and L units cancel, leaving us with both units of mL Another Example ● The mass of the Earth’s moon is 7.35 x 1022 kg. What is the mass expressed in  nanograms, ng? ● We want ng; we have 7.35 x 1022 kg  ● Convert to kilograms to grams, and then grams to nanograms Metric­English Conversions  ● The English system is still very common in the United States ● We often have to convert between English and metric units  Metric­English Conversions, Continued  ● The length of an American football field, including the end zones, is 120 yards.  What is the length in meters? ● Convert 120 yd to meters (given that 1 yd= 0.914 m) Metric­English Conversions, Continued  ● A half­gallon carton contains 64.0 fl oz of milk. How many millimeters of milk are  in a carton? Another English­ Metric Problem ● A marathon is 26.2 miles. What is the distance in kilometers? ● Step 1: We want km ● Step 2: We write down the given:  26.2 mi. ● Step 3: We apply a unit factor (1 km = 0.62 mi) and round to 3 sig figs Compound Units ● Some measurements have a ratio of units ● For example, the speed limit on many highways is 55 miles per hour. How could  you convert this to meters per second? ● Convert one unit at a time using unit factors ○ First, miles ­­­­­­> meters ○ First, hours ­­­­­­­> seconds Compound Unit Problem  ● A motorcycle is traveling at 105 km/hour. What is the speed in meters per  second? ○ We have km/h, we want m/s ○ Use 1 km = 1000 m and 1 h = 3600 seconds        Chemistry Connection: The Olympics ● While the United States still uses English units of measure (mile, gallon, pounds), most of the rest of the world uses the metric system. ● The distances in Olympic events are in metric units: 100­m dash; 30­km cross­ country skiing; 3000­m steeplechase. ● The 1600­m run is approximately 1 mile in length.        Critical Thinking: World Trade Center ● When discussing measurements, it is critical that we use the proper units ● The World Trade Center footprint was 150 feet square, not 150 square feet. ● NASA engineers mixed metric and English units when designing the Mars  Climate Orbiter ● The engineers used kilometers rather than miles. ● The spacecraft approached too close to the Martian surface and burned up in the atmosphere.        The Percent Concept ● A percent, %, expresses the amount of a single quantity compared to an entire  sample. ● A percent is a ratio of parts per 100 parts. ● The formula for calculating percent is shown below: 


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