Why Chemistry? Why Chemistry?

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1 Why Chemistry? Engineering Plastics - How does structure affect: Elasticity, Processability Composite Materials What additives make a material stronger, lighter, faster? Pulp and Paper What innovations can be made in the chemical bleaching processes? CHEM 100 F07 1 Medicine Why Chemistry? How does this drug interact with that enzyme? How does this chemical imbalance affect physiology? Environment How does an increased ph affect river life? How can we remove environmental toxins? Not create them in the first place? CHEM 100 F07 2 1

2 What is Chemistry? The science of matter All things are made of atoms Bridge between macroscopic and microscopic It s hard! How do you work with things that you can t see? You must be multifaceted! We are here to help! Strategies! Tips! Success! CHEM 100 F07 3 How do we deal with numbers? (Ch. 2) Units(!) and Measurements [2.1, 2.5] Significant Figures (sig figs, sf) [2.3] Scientific Notation [2.2] Conversions and unit-factor method [2.6, 7] Problem-solving and practice [2.7, 8, 10] Density Problems [2.9] CHEM 100 F07 4 2

3 Units and Measurements SI (Système International) units Quantity Length Mass Time Temperature Electric current Amount of Substance Luminous Intensity Unit Meter Kilogram Second Kelvin Ampere Mole Candela Symbol m kg s K A mol cd CHEM 100 F07 5 Measurements and Numbers In the lab, what instruments should you choose? How do you write numbers off of equipment? CHEM 100 F07 6 3

4 Significant Figures Significant figures (sig figs) indicate the amount of certainty and precision in a measurement. If you are asked to weigh out ten grams of baking soda, how carefully did you weigh out those ten grams? CHEM 100 F07 7 Rules for Sig Figs 1. All non-zero integers are significant. Examples: 96,485 Coulombs/mol 26.4 miles 2.2 lbs in a kilogram 98.6 F CHEM 100 F07 8 4

5 Rules for Sig Figs 2. All exact numbers have an unlimited amount of significant figures. 1 inch = 2.54 cm exactly 1 dozen = 12 items exactly CHEM 100 F07 9 Rules for Sig Figs 3. Zeros: a) Leaders: Leading zeros precede all non-zero digits. They ARE NOT significant. b) Captives: Captive zeros lie between non-zero digits, or two significant figures. They ARE significant. c) Followers: Trailing zeros fall after a non-zero digit and a decimal point. They ARE significant. CHEM 100 F

6 Rules for Sig Figs Problem - Determine the number of sig figs in each measurement g 37.2 C kg 1200 lbs hrs 5.33 ft moles ml CHEM 100 F07 11 More Rules for Sig Figs 4. Rounding: If the number is 5 or greater, round up. If it is less than 5, round down. Round the following numbers to 4 sig figs: Examples: g mol g Problems: g kg ml CHEM 100 F

7 Sig Figs and Scientific Notation Consider the following numbers: 250 ml g (rounded to 4 sf) How many sig figs in the first? Are you sure? How can you be sure? CHEM 100 F07 13 Scientific Notation Using Scientific Notation gives all (and only) the significant digits! A number expressed in scientific notation contains the following components: x 10 4 kg CHEM 100 F

8 Scientific Notation Perform the following steps to write a number in scientific notation: 1. Move the decimal point (put one in, if needed) until there is one digit in front of the decimal point. a) Count the number of jumps that the decimal place moves. b) If you move the decimal place to the left, the exponent is positive. If you move the decimal place to the right, the exponent is negative. 2. Write that number x 10 n, where n is number of jumps, and has the appropriate sign. 3. Verify that the number has the correct number of sig figs indicated/needed. CHEM 100 F07 15 Scientific Notation - Examples Use scientific notation to express the following numbers to 3 sig figs m/s amu CHEM 100 F

9 Scientific Notation - Problems Use scientific notation to express the following numbers to 4 sig figs L mol g CHEM 100 F07 17 Sig Figs and Calculations When performing calculations with experimental data, the sig figs in the answer must reflect the sig figs in the initial values. There are two sets of rules to follow: Multiplication/Division Addition/Subtraction CHEM 100 F

10 Sig Figs and Multiplication 1. Count the number of sig figs in each measurement or factor. 2. Perform the calculation. 3. Round your answer to the same number of sig figs as in the smallest measurement or factor. Examples: x = x 10-3 = CHEM 100 F07 19 Sig Figs and Addition 1. Line up the numbers vertically with the decimal point in the same place. 2. Perform the calculation. 3. Inspect the decimal places of each measurement or factor. The answer should have the same number of decimal places as the lowest number of decimal places from the measurements or factors. Examples = = CHEM 100 F

11 g g g = Practice! = 41.8/13.80 = x 10 2 x 5.89 x 10-1 =! " # x $ % & = CHEM 100 F07 21 Sig Figs and Calculations What if you have both addition/subtraction and multiplication/division? Example In the lab, you determine the density of a piece of copper to be 8.74 g/ml. The known density of copper is g/ml. What is the percent difference between the two values? Percent Difference = $ actual - experimental % & x 100% actual! " # CHEM 100 F

12 Percent Difference In the lab, you determine that the volume of a Chromium atom is 7.31 cm 3. The actual value is 7.23 cm 3. Determine the % difference between your value and the accepted value. CHEM 100 F07 23 Unit Conversions Sometimes it is necessary to convert from one set of units to another. Example: A matchbox (toy) car weighs ~200 g, a 2007 Toyota Camry Hybrid weighs 1,653,000 g. Is the gram an appropriate unit for the matchbox car? Is the gram an appropriate unit for the Camry? Powers of Ten Multiple Prefix Symbol 10 3 kilo k 10-3 milli m 10-6 micro µ 10-9 nano n CHEM 100 F

13 Unit Conversions How do we use the values in the table? How many ml are in 1 L? (a lot or a little?) How many kg are in 1 g? (a lot or a little?) How do we put this in a calculator? 1 x EE -3 CHEM 100 F07 25 Unit Conversions The gram is an base unit (it is a unit of mass), but it is not the best power of ten! Problem: Convert the mass of the Camry into kg. A unit factor or conversion factor 1,653,000 g x 1 kg 10 3 = 1,653 kg g Check: Should the number have gotten bigger or smaller? Problem: Determine the number of milliliters in a 2-L bottle of soda. CHEM 100 F

14 Unit Conversions Problem: The size of atoms is often given in the units of Angstroms (Å), which is equal to 1 x m. The radius of an iron atom is 2.86 Å. What is this in meters? In nanometers? CHEM 100 F07 27 Unit Conversions - Unit Factor Method Unit conversions can occur between different systems of measurements. Problem: A cake recipe calls 3 cups of milk. You only have an 4 oz glass. How many glasses of milk should you measure out? (1 cup = 8 oz) CHEM 100 F

15 Unit Conversions Problem: I am ft tall. How tall is that in cm? CHEM 100 F07 29 Unit Conversions Problem: Convert x 10 8 m/s to miles/hr (1 km = mi) CHEM 100 F

16 Unit Conversions Special Care must be given when converting between units that are raised to a power. Problem: Acceleration due to gravity is 9.8 m/s 2. What is gravity in miles/hr 2? CHEM 100 F07 31 Problem Solving How do we pick apart a problem, to prepare to solve it? Xylitol, a common sugar substitute in gum and candy, has been found in one recent study to be lethal to dogs. The LD 50 is 22.3 g xylitol/kg dog. Your 50.0 lb. dog eats 12 pieces of sugar-free gum (each piece has 0.67 g). Is your dog in danger? CHEM 100 F

17 Density The Density of a substance can be thought of as unit conversion factor. Definition: The volume occupied by a certain mass of a substance. Units: g/ml or g/cm 3 (1 ml = 1 cm 3 ) Examples: lead = g/ml iron = 7.87 g/ml blood (37 C) = g/ml water (37 C) = g/ml CHEM 100 F07 33 Density Problems The density of gold is 19.3 g/cm 3. What volume does 59 mg occupy? Diamond has a density of g/cm 3. The mass of diamonds is often measured in carats, where 1 carat = g. What is the volume of a 1.50-carat diamond? CHEM 100 F

18 More Density Problems Determine the density of benzene, if 166 g of the liquid fills a graduated cylinder to ml. What is the mass of 2.0L of isopropanol (rubbing alcohol), if the density is g/ml? CHEM 100 F07 35 More Density Problems I have a 29-gallon fish tank at home. If I fill it up with 28.5 gallons of water, how much does it weigh, in kg? (d H2O = g/ml, 1 gal = L) CHEM 100 F

19 More Density Problems My laptop is encased in aluminum, and weighs 5.7 lbs. If the density of aluminum is 2.70 g/cm 3, what volume does my laptop occupy? (1 lb = g) According to the specs from Apple, the laptop measures 13.7 in wide x 9.5 in deep x 1.1 in thick. What is the volume of my laptop? Is my laptop made of pure aluminum? CHEM 100 F