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Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
97
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 10
10.4 Empirical and Molecular Formulas
Recall that every chemical compound has a definite composition—a
composition that is always the same wherever that compound is
found. The composition of a compound is usually stated as the percent by mass of each element in the compound.
Percent composition The percent of an element in a compound
can be found in the following way.
% by mass of an element ϭ
ϫ 100
The example problem below shows you how to determine the
percent composition of a compound, which is the percent by mass
of each element in the compound.
Example Problem 10-7
Calculating Percent Composition
Determine the percent composition of calcium chloride (CaCl 2 ).
First, analyze the information available from the formula. A mole of
calcium chloride consists of one mole of calcium ions and two
moles of chloride ions.
Next, gather molar mass information from the atomic masses on the
periodic table. To the mass of one mole of CaCl 2 , a mole of calcium
ions contributes 40.078 g, and two moles of chloride ions contribute
2 ϫ 35.453 g ϭ 70.906 g for a total molar mass of 110.984 g/mol
for CaCl 2 .
Finally, use the data to set up a calculation to determine the percent
by mass of each element in the compound. The percent by mass of
calcium and chlorine in CaCl 2 can be calculated as follows.
% Ca in Ca Cl 2 ϭ
ϫ 100 ϭ 36.112% Ca
% Cl in CaCl 2 ϭ
ϫ 100 ϭ 63.888% Cl
70.906 g Cl
ᎏᎏ
110.984 g CaCl 2
40.078 g Ca
ᎏᎏ
110.984 g CaCl 2
mass of element in 1 mol compound
ᎏᎏᎏᎏ
molar mass of compound
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