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98 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 10
As a check, be sure that the percentages add up to 100%. In this
case, the percentages add up to 100.000%.
Practice Problems
21. Calculate the percent composition of aluminum oxide (Al 2 O 3 ).
22. Determine the percent composition of magnesium nitrate,
which has the formula Mg(NO 3 ) 2 .
23. Calculate the percent oxygen in potassium chlorate (KClO 3 ).
24. Calculate the percent nitrogen in ammonium hexacyanoiron(II),
which has the formula (NH 4 ) 4 Fe(CN) 6 .
25. Acetylene gas has the molecular formula C 2 H 2 . How does the
percent composition of acetylene compare with that of benzene
(C 6 H 6 )?
Empirical formulas You can use percent composition data to
help identify an unknown compound by determining its empirical
formula. The empirical formula is the simplest whole-number ratio
of atoms of elements in the compound. In many cases, the empirical
formula is the actual formula for the compound. For example, the
simplest ratio of atoms of sodium to atoms of chlorine in sodium
chloride is 1 atom Na : 1 atom Cl. So, the empirical formula of
sodium chloride is Na 1 Cl 1 , or NaCl, which is the true formula for
the compound. The following example problem will show you how
to determine empirical formulas.
Example Problem 10-8
Empirical Formula from Percent Composition
The percent composition of an unknown compound is found to be
38.43% Mn, 16.80% C, and 44.77% O. Determine the compound’s
empirical formula.
Because percent means “parts per hundred parts,” assume that you
have 100 g of the compound. Then calculate the number of moles of
each element in the 100 g of compound. The number of moles of
manganese may be calculated as follows.
moles of Mn (in 100 g) ϭ grams Mn (in 100 g) ϫ
1 mol Mn
ᎏᎏ
molar mass Mn
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