Welcome to AP Chemistry Significant figures Meaningful digits in a MEASUREMENT Exact numbers are counted, have unlimited significant figures If it is measured or estimated, it has sig figs. If not it is exact. All numbers except zero are significant. Some zeros are, some aren’t Which zeroes count? In between other sig figs does Before the first number doesn’t After the last number counts if: it is after the decimal point the decimal point is written in 3200 2 sig figs 3200. 4 sig figs Doing the math Multiplication and division, same number of sig figs in answer as the least in the problem Addition and subtraction, same number of decimal places in answer as least in problem. More Preliminaries Scientific Method Metric System Uncertainty Scientific method. A way of solving problems Observation- what is seen or measured Hypothesis- educated guess of why things behave the way they do. (possible explanation) Experiment- designed to test hypothesis leads to new observations, and the cycle goes on Scientific method. After many cycles, a broad, general explanation is developed for why things behave the way they do Theory Also regular patterns of how things behave the same in different systems emerges Law Laws are summaries of observations Scientific method. Theories have predictive value. The true test of a theory is if it can predict new behaviors. If the prediction is wrong, the theory must be changed. In Short: Theory- why Law - how Observations Hypothesis Theory (Model) Modify Experiment Prediction Law Experiment Metric System Every measurement has two parts Number and a Scale (unit) SI system (le Systeme International) based on the metric system Prefix + base unit Prefix tells you the power of 10 to multiply by - decimal system -easy conversions Metric System Base Units Mass - kilogram (kg) Length- meter (m) Time - second (s) Temperature- Kelvin (K) Electric current- ampere (amp, A) Amount of substance- mole (mol) Prefixes giga- mega - M kilo deci centi milli micro nano G k d c m m n 1,000,000,000 109 1,000,000 106 1,000 103 0.1 10-1 0.01 10-2 0.001 10-3 0.000001 10-6 0.000000001 10-9 Deriving the Liter 3 Liter is defined as the volume of 1 dm V = length x width x height Mass and Weight Mass is measure of resistance to change in motion Weight is force of gravity on a mass Fw = mass x g (g = 9.8 m/s2) Sometimes used interchangeably Mass can’t change, weight can Uncertainty Basis for significant figures All measurements are uncertain to some degree Precision- how repeatable Accuracy- how correct - closeness to true value. Better precision implies better accuracy You can have precision without accuracy You can’t have accuracy without precision Uncertainty 2 Types of Error: Random error - equal chance of being high or low- addressed by averaging measurements - expected Systematic error- same direction each time Want to avoid this type Dimensional Analysis Using the units to solve problems Dimensional Analysis Use conversion factors to change the units Conversion factors = 1 1 foot = 12 inches (equivalence statement) 12 in = 1 = 1 ft. 1 ft. 12 in 2 conversion factors multiply by the one that will give you the correct units in your answer. Examples The speed of light is 3.00 x 108 m/s. How far will a beam of light travel in 1.00 ns? 1.00 ns 1 x 10-9s 1 ns 3.00 x 108 m 1s = 0.300 m Dealing with Two Units If your pace on a treadmill is 65 meters per minute, how many seconds will it take for you to walk a distance of 8450 feet? What about Square and Cubic units? Use the conversion factors you already know, but when you square or cube the unit, don’t forget to cube the number also! Best way: Square or cube the ENITRE conversion factor Example: Convert 4.3 cm3 to mm3 4.3 cm3 10 mm ( 1 cm 3 ) = 4.3 cm3 103 mm3 13 cm3 = 4300 mm3 Learning Check A Nalgene water bottle holds 1000 cm3 of water. How many cubic decimeters is that? Solution 1000 cm3 1 dm 10 cm ( 3 ) = 1 dm3 So, a dm3 is the same as a Liter ! A cm3 is the same as a milliliter. Temperature and Density Temperature A measure of the average kinetic energy Different temperature scales, all are talking about the same height of mercury. 100ºC = 212ºF 0ºC = 32ºF 0ºC 100ºC 212ºF 32ºF oF = 1.8(oC) + 32 oC = (oF – 32) / 1.8 K = oC +273 0ºC 100ºC 212ºF 32ºF Density Ratio of mass to volume D = m/V Useful for identifying a compound Useful for predicting weight An intrinsic property- does not depend on the quantity of the material Density Problem An empty container weighs 121.3 g. Filled with carbon tetrachloride (density = 1.53 g/cm3 ) the container weighs 283.2 g. What is the volume of the container? V=m/D m = 161.9 g D = 1.53 g/cm3 V = 161.9 g /(1.53 g/cm3) = 106 cm3 Density Problem A 55.0 gal drum weighs 75.0 lbs. when empty. What will the total mass be when filled with ethanol? density of ethanol is 0.789 g/cm3 1 gal = 3.78 L 1 lb = 454 g 55.0 gal | 3.78 L | 1000 mL | 1 cm3| 0.789 g | 1 lb = 361 lb | 1 gal | 1L | 1 mL | 1 cm3 | 454 g 361 lb + 75.0 lb = 436 lb Grams to Mols to Atoms Use molar mass to convert from grams to mols or from mols to grams Conversions involving atoms, molecules, or ions: use Avogadro’s number 6.02 x 1023 particles/mol % Error = Accepted value – Experimental value x 100 Accepted value Negative % errors are OK