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