Percent Composition and
Empirical Formula
Percent Composition General Strategy
• Convert whole number ratio (moles) to mass
percent
Formula  Mass Percent
Percent Composition Steps
• Multiply each whole number subscript by the
appropriate molar mass.
• Divide the part of each molecule and divide by
the total.
• Multiply by 100%.
Example 1
• What is the percentage of each element in
SO2?
Example 2
• What is the percentage of each element in
Ca(OH)2 ?
Empirical Formula General Strategy
• Convert mass percents to whole number ratio
(moles)
Mass Percent  Formula
Empirical Formula Steps
• Assume each given percentage is a mass
• Divide each given percentage by the
appropriate molar mass.
• Divide all values by the smallest number of
moles to obtain a whole number ratio
Empirical Formula
• Definition: The formula with the simplest
whole number ratio.
Decimals you should not just round:
• 0.5 = ½
• 0.25 = ¼
• 0.33 = 1/3
• 0.66 = 2/3
Molecular Formula
• Definition: A molecular formula may be
simplified to an empirical formula.