Chemistry Reference
In-Depth Information
Why was the liter eliminated from the International System of Mea-
surements? Well, one of the goals of the conference was to cut down on
the number of existing units and start off with the fewest number of base
units that were needed to measure essentially all of the known physical
quantities. Because volume can be expressed in cubed units of length, there
really is no reason to support special units of volume. Following are the
seven SI base units.
These base units can
be combined with prefixes
in order to derive larger or
smaller quantities. For ex-
ample, we combine the
prefix “kilo-” with the base
unit “meter” to get one
“kilometer,” which has a
value of 1000 meters. The
SI prefixes are shown on
page 44.
If you are asked to
convert between units of
the same quantity, simply
pay attention to what the
prefixes stand for. If asked
to convert 3.45 kilograms
to grams, keep in mind that “kilo-” means thousand, so one kilogram must
equal one thousand grams. This makes the conversion as simple as the work
shown here:
SI Base Units
Property
Unit
Symbol
length
meter
m
time
second
s
mass
kilogram
kg
temperature
Kelvin
K
electric current
ampere
A
amount of substance
mole
mol
luminous intensity
candela
cd
Figure 2-1c
3.45 kg × 1000 g/1 kg = 3450 g
Notice that we multiply the original quantity (3.45 kg) by a ratio of
1000 g/1 kg. By orienting the ratio in such a way that the symbol “kg” is in
the denominator, we're able to “cross-cancel” the unwanted unit (kg), leav-
ing us with the desired unit (g).
You can check your answer by reasoning, “If 1 kilogram is 1000 grams,
then 3 kilograms would be 3000 grams.” Your answer seems reasonable. If
you find this difficult, you may want to skip ahead to Lesson 2-5, where we
will be covering conversions in greater detail.
