Percents come up several
times in chemistry. Remember that the key is to change a percent
to a number
before calculating with it and, if the answer is to be stated as a percent, convert the
number to a percent before giving the answer.
- When measuring a sample for its constituent parts, the amounts of each
part
are often stated as %s -- you'll see this in percent abundance of isotopes and in
percent composition of compounds. Don't forget: part / whole.
For example:
A sample of lead was tested in a mass spectrometer, and four isotopes
were found along with their % abundances:
204 at 1.4%, 206 at 24.1%, 207 at 22.1% and 208 at 52.4%.
How do we read these? 1.4% of the sample was isotope 204, 24.1% of
the sample was 206, etc. Notice that the percents add up to 100. (All
the parts together should total up to the whole!)
- When conducting an experiment to synthesize a chemical compound, you'll
compare the amount you should get (according to the theory of how
chemicals bind together) to the actual amount you
did get from your experiment --
percent yield = experiment / theory.
For example:
Suppose we know that if we take formic acid and
geraniol, we can make a synthetic rose perfume. If we start with
1000.0 g of geraniol added to formic acid, the theory of chemical
reactions (stoichiometry!) can be used to calculate
a theoretical yield of 1182.2 g of the rose essence.
Our experiment actually produces 871.2 g. What is the percent yield?
experiment / theory = 871.2 / 1182.2 = .736931145 = 73.69% to four sig figs.
- When working with measurements there is often some associated error
-- usually measured as
percent error:
For example, suppose we test a new thermometer for accuracy by using it to find the boiling point
of pure water.
The boiling point of pure water is 212°F but the thermometer measures
212.9°F. What is the % error of the reading?
which is .425% to
three sig figs. Note the absolute values; that's why the - sign was dropped.