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28 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER
3
e. aluminum foil
f. the substances listed on the periodic table
g. calcium chloride pellets used to melt ice
A given compound is always composed of the same elements in the
same proportion by mass. This fact is known as the law of definite
proportions. For example, 100.00 g of H 2 O always contains 11.19 g
of hydrogen and 88.81 g of oxygen, no matter where the water came
from. Compounds are often identifiable from their percentage composition, the percent by mass of each element in a compound. The
percent by mass of each element in a compound may be found by
using the following equation.
Percent by mass of an element (%) ϭ
ϫ 100%
For example, suppose you break a compound down into its elements
and find that 25.00 g of the compound is composed of 6.77 g of tin
and 18.23 g of bromine. The percent by mass of tin in the compound
can be determined as follows.
Percent by mass of tin ϭ
ϫ 100%
Percent by mass of tin ϭ
ϫ100% ϭ 27.1% tin
Practice Problems
10. Follow the procedure described above to determine the percent
by mass of bromine in the compound discussed above. What is
the sum of the percents of the two elements?
11. A 134.50-g sample of aspirin is made up of 6.03 g of hydrogen,
80.70 g of carbon, and 47.77 g of oxygen. What is the percent
by mass of each element in aspirin?
12. A 2.89-g sample of sulfur reacts with 5.72 g of copper to form
a black compound. What is the percentage composition of the
compound?
6.77 g
ᎏ
25.00 g
mass of tin
ᎏᎏ
mass of compound
mass of element
ᎏᎏ
mass of compound