The SI System
The Système Internationale (International System), SI,
more commonly known as the metric system, is the only system of
measurement that you’ll see on the SAT II Chemistry test. Here’s
a quick refresher course on how to use this system.
Standard Prefixes
The metric system is fairly straightforward. The table
of prefixes below is only partial, but it includes the ones that
you need to be familiar with for the test.
| Prefix |
Power |
Meaning |
Examples of measurements |
| nano (n) |
10-9 |
one-billionth |
nanometer (nm): wavelength of light |
| micro (m) |
10-6 |
one-millionth |
micrometer (mm): width of
a hair |
| milli (m) |
10-3 |
one-thousandth |
milliliter (mL): volume of acid in burette |
| centi (c) |
10-2 |
one-hundredth |
centimeter (cm): length of paper |
| deci (d) |
10-1 |
one-tenth |
deciliter (dL): amount of liquid |
| kilo (k) |
103 |
one thousand times |
kilogram (kg): your weight |
Also useful to know are the units in the table below.
Most of them will probably be familiar to you.
| What is being measured |
Common units |
| length |
meter (m) |
| mass |
gram (g) |
| volume |
liter (L) or cm3 |
| temperature |
degree Celsius (˚C) and kelvin
(K) |
| time |
second (s) |
| pressure |
kilopascal (kPa); atmosphere
(atm); mmHg |
| energy |
joules ( J); calorie (cal) |
| amount of substance |
mole (mol) |
Scientific Notation
This is an easy way to express really large or really
small numbers conveniently. The general format for numbers expressed
this way is
some number
10some power
For instance, 6.022
10
23 is
really big, and 3.00
10
-6 is
really small. Notice that the proper position for the decimal is
to the right of the first nonzero digit. If you must move the decimal
to get it into this position, moving the decimal to the left makes
the exponent appear larger, while moving decimal to the right makes
the exponent appear smaller. For example, 0.000567 in scientific
notation would be 5.67
10
–4.
You need to be able to handle numbers of this sort without
a calculator. Basically, you need to remember the following. For multiplication,
add exponents, and for division, subtract exponents.
To get the log of a value, raise it to the power
of ten. This is mostly useful for pH calculations. Now
try some problems.
Example
(4.5
10
5)(3.0
10
8).
Explanation
The answer is 1.35
10
14 (or
rounded, 1.4
10
14).
In solving this, think: 3
5 = 15, and then
add the exponents: 5 + 8 = 13. Move the decimal to the right of
the first nonzero digit, or one place to the left.
Example
Try another one:
.
Explanation
The answer is 3.4
10
-12.
In solving this, think: 6.8/2 = 3.4, and then subtract the exponents:
(-2) - (10) = -12.
Example
Let’s try another: Find the log of 1.0
10
-7.
Explanation
The answer is -7. The thought process is as follows. The
log of 1.00 is 0. The log of 10-7 is just
the power of 10.
Temperature Conversions
The only two temperature scales that are needed for the
SAT II Chemistry test are the Celsius scale and the Kelvin scale.
One degree on the Celsius scale is the same increment as 1 kelvin
on the Kelvin scale.
Celsius scale: This is the scale used in
the chemistry laboratory for most experiments. The freezing point
of water is 0ºC, and the boiling point of water is 100ºC. This was
the original metric standard for temperature.
Kelvin scale: This is the scale used for
working through gas law problems. There are no negative numbers
on this scale. At 0K, all motion theoretically ceases.
Calculations Involving Metric Measurements (Dimensional Analysis)
Dimensional analysis offers an easy way
to solve problems using conversion factors and unit cancellations. Conversion
factors are ratios that equal 1. You know many of these ratios
of equivalencies from everyday living. For example, 1 gallon equals
4 quarts, 12 inches equals 1 foot, etc. This is a useful technique
for calculations that might come up on the test, so work through
the following problems to practice it.
Example
How many inches tall is a person who is 5 feet, 4 inches
tall?
Explanation
Example
How many milliliters would there be in 3.5 liters of soda?
Explanation
You’ll have to do plenty of conversions like the one above
to solve problems on the exam. Be sure that you are familiar with
all the metric prefixes listed earlier so that you can be successful
when you need to convert numbers.
Density
Density is a complex unit. It is defined as mass per unit
of volume:
All pure substances have a unique density at a given temperature.
Density is an intensive physical property, meaning that it does
not change with sample size. Usually the solid form of a pure substance
is denser than the liquid form of the same substance. This makes
sense because in most solids, the particles are much closer together
than in their liquid counterparts.
Typical units for density of solids and liquids are grams
per milliliter or grams per cubic centimeter. (Remember: 1 cm3 =
1 mL.) Typical units for density of gases are grams per liter.
Example
Find the density of a substance that has a mass of 45.0
g and a volume of 3.0 mL.
Explanation
Example
What would be the mass of a substance that occupies a
space of 2.0 cm3 and has a density of 7.5
g/cm3?
Explanation
. Rearrange the
equation to solve for mass:
M =
D
V.Then
M = (7.5 g/cm3)(2.0
cm3) = 15 g
