(^46) › STEP 4. Review the Knowledge You Need to Score High
Units and Measurements
Almost all calculations in chemistry involve both a number and a unit. One without
the other is useless. Every time you complete a calculation, be sure that your units have
canceled and that the desired unit is written with the number.
Always show your units!
Units
The system of units used in chemistry is the SI system (Système International), which is
related to the metric system. There are base units for length, mass, etc. and decimal pre-
fixes that modify the base unit. Since most of us do not tend to think in these units, it is
important to be able to convert back and forth from the English system to the SI system.
These three conversions are useful ones, although knowing the others might allow you to
simplify your calculations:
mass: 1 pound = 0.4536 kg (453.6 g)
volume: 1 quart = 0.9464 dm^3 (0.9464 L)
length: 1 inch = 2.54 cm (exact)
As shown above, the SI unit for volume is the cubic meter (m^3 ), but most chemists use
the liter (L, which is equal to 1 cubic decimeter [dm^3 ]) or milliliter (mL). The appendixes
list the SI base units and prefixes, as well as some English–SI equivalents.
We in the United States are used to thinking of temperature in Fahrenheit, but most
of the rest of the world measures temperature in Celsius. On the Celsius scale, water freezes
at 0°C and boils at 100°C. Here are the equations needed to convert from Fahrenheit to
Celsius and vice versa:
°=°−
°= °+
C
5
9
[F 32 ]
F
9
5
(C)3 2
Many times, especially in working with gases, chemists use the Kelvin scale. Water
freezes at 273.15 K and boils at 373.15 K. To convert from Celsius to Kelvin:
K=°C 273.15+Absolute zero is 0 K and is the point at which all molecular motion ceases.
The density of a substance is commonly calculated in chemistry. The density (D) of
an object is calculated by dividing the mass of the object by its volume. (Some authors will
use a lowercase d to represent the density term; be prepared for either.) Since density is
independent of the quantity of matter (a big piece of gold and a little piece have the same
density), it can be used for identification purposes. The most common units for density in
chemistry are g/cm^3 or g/mL.Measurements
We deal with two types of numbers in chemistry—exact and measured. Exact values are just
that—exact, by definition. There is no uncertainty associated with them. There are exactly
12 items in a dozen and 144 in a gross. Measured values, like the ones you deal with in
the lab, have uncertainty associated with them because of the limitations of our measuring
instruments. When those measured values are used in calculations, the answer must reflect
that combined uncertainty by the number of significant figures that are reported in the
final answer. The more significant figures reported, the greater the certainty in the answer.