Empirical Formula vs.
Molecular Formula
• Empirical formula: the formula for a
compound with the smallest whole-number
mole ratio of the elements
• Molecular formula: formula for a compound
with the actual number of atoms of each
element in a molecule or formula unit of the
substance
• What is the difference between a molecule
and a formula unit?
Empirical Formula vs.
Molecular Formula, continued……
• Example of the difference: Benzene is
composed of 6 carbon and 6 hydrogen (C6H6).
• What is the ratio of carbon-to-hydrogen in
benzene?
• If we reduce the compound formula to its
“smallest whole-number mole ratio of the
elements” (empirical formula), we are left
with CH
• Acetylene’s molecular formula is C2H2. What
is its empirical formula?
Calculating Empirical Formula From
Percent Composition
1. If given percent composition of a compound,
make the percent of each element that
element’s mass in grams, so the whole
compound has a molar mass of 100 grams.
2. Convert the mass of each element to moles.
3. If the answers (moles) are not whole numbers,
divide all answers by the smallest number of
moles present.
4. If still not whole numbers, multiply all answers
by the smallest factor that will give you whole
numbers.
Calculating Empirical Formula From
Percent Composition, continued…..
• Examples: P.349 (144, 147)
• Practice Problems (p.333) --- reference
Example Problem on p.332 for needed
guidance