Welcome to AP Chemistry. Scientific method A logical way of solving problems Three steps –Making observations –Formulate hypothesis –Perform experiments.

Welcome to AP Chemistry

Scientific method

• A logical way of solving problems• Three steps

– Making observations– Formulate hypothesis– Perform experiments

• Hypothesis is an educated guess (possible explanation)

• Experiment is designed to test hypothesis and leads to new observations

Scientific method• After many cycles, a broad, generalized

explanation is developed for why things behave the way they do (Theory explains why)

• Also regular patterns of how things behave the same in different systems emerges (Law explains how)

• Laws are summaries of observations

Observations

Hypothesis

Experiment

Law

Theory(Model)

Prediction

Experiment

Modify

Metric System• Every measurement has two parts

– Number– Unit

• SI system (le Systeme International) is based on the metric system

• Prefix + base unit (Prefixes found on p. 10)– Prefixes are based on powers of 10

Mass and Weight

• Mass is measure of resistance to change in motion

• Weight is force of gravity

• Sometimes used interchangeably

• Mass can’t change, weight can

Uncertainty of Measurement• Basis for significant figures

– All measurements are uncertain to some degree

• Precision is how repeatable the measurement is

• Accuracy is how close to true value.

• Random error - equal chance of being high or low- addressed by averaging measurements (It is expected)

Uncertainty of Measurement• Systematic error- same direction each

time

• Better precision implies better accuracy– Precision can occur without accuracy– Accuracy cannot occur without precision

Significant figures

• All measurements have some degree of uncertainty

• Exact numbers are counted, have unlimited significant figures

• If it is measured or estimated, it has sig figs.

• All numbers except zero are significant.• Some zeros are significant, some aren’t

Which zeroes count?

• In between other sig figs, zeros count• Before the first non-zero, zeros don’t

count• After the last number, zeros count if

it is after the decimal point• 3200 2 sig figs

• 3200. 4 sig figs

Sig fig math

• In multiplication and division, the answer must have the same number of sig figs as the number with the least sig figs in the problem.

• In addition and subtraction, same number of decimal places must be used in the answer as the number with the fewest sig figs.

Dimensional Analysis• Use conversion factors to change units• Conversion factors = 1 (ALWAYS!)• 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• 11 yards = 2 rod• 40 rods = 1 furlong• 8 furlongs = 1 mile• The Kentucky Derby race is 1.25 miles. How

long is the race in rods, furlongs, meters, and kilometers?

• A marathon race is 26 miles, 385 yards. What is this distance in rods, furlongs, meters, and kilometers?

• The speed of light is 3.00 x 108 m/s. How far will a beam of light travel in 1.00 ns?

Temperature

• A measure of the average kinetic energy

• Use three different temperature scales, but all are talking about the same height of mercury. (Celsius, Kelvin, Fahrenheit)

0ºC 32ºF

0ºC = 32ºF

100ºC 212ºF

100ºC = 212ºF

0ºC 32ºF

0ºC = 32ºF

100ºC 212ºF0ºC 32ºF

100ºC = 212ºF0ºC = 32ºF

100ºC = 180ºF

100ºC 212ºF0ºC 32ºF

100ºC = 212ºF0ºC = 32ºF

100ºC = 180ºF1ºC = (180/100)ºF1ºC =

9/5ºF

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 amount 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?

Classification of Matter• Pure substances have constant composition

– Elements cannot be broken into simpler substances– Compounds have constant composition and can be broken down

• Separation–Filtration removes a solid from a liquid–Distillation uses heat to separate gases–Chromatography uses the mobile phase (liquid or gas) to remove from a stationary phase (solid) substance