Empirical Formula
Empirical: based on observation and
experiment
Empirical Formula
• The lowest, whole number ratio of the
atoms in a compound
• The empirical formula of a compound
does not always equal the molecular
formula
– Example: Hydrogen Peroxide
» Molecular Formula = H2O2
» Empirical Formula = HO
Ionic Formula
• Ionic formula always equals empirical
formula
• Ionic compounds are always simple,
whole-number ratios of elements
• Examples:
– FeS
– Ammonium Phosphate
– CaCO3
Determining Empirical Formula
• Example: A compound has a percent
composition of 27.29% carbon and
72.71% oxygen. What is the compound’s
empirical formula?
STEP ONE: Assume sample size is 100g
STEP TWO: Determine how many grams
of each element are present using percent
composition
» 27.29g C
» 72.71g O
STEP THREE: Determine the number of
moles of each element in the sample
Moles carbon = 27.29 g x 1 mol C = 2.27 moles C
1
12.0 g
Moles oxygen = 72.71 g x 1 mol O = 4.54 moles O
1
16.0 g
STEP FOUR: Convert the ratio of moles to
the lowest whole number ratio by dividing
each number by the lowest number of
moles present
C = 2.27 mol = 1
2.27 mol
O= 4.54 mol = 2
2.27 mol
Therefore, the empirical formula of this
compound = CO2
Example #2
If 2.5 g of Al is heated with 5.28g of F, what
is the EF of the resulting compound?
2Al + 3F2
2AlF3
Empirical Formula
2Al + 3F2
2.50g
2AlF3
5.28g
Law of Conservation of Mass =
Total mass of the compound = 7.78g
Al = 2.50g/7.78g x 100% = 32.1%
F = 5.28g/7.78g x 100% = 67.9%
Change into grams
2.50g Al and 5.28g F
Al
32.1%
F 67.9%
32.1g
67.9g
Determine how many moles of each you have
Al
F
Molecular Formula
• Either the same as empirical formula or a
simple, whole number multiple of its
empirical formula
• Example: Benzene
» Empirical = CH
» Molecular = C6H6
• Example: Methanol
» Empirical = CH4O
» Molecular = CH4O
Determining Molecular Formula
• From empirical formula, empirical formula
mass (efm) can be determined
• Example: HO = 17.0 g/mol
• Molar mass is determined experimentally
• Example: 34.0 g/mol
• Number of empirical formula units can be
determined by these two values
Molar Mass = Empirical Formula Multiplier
efm
• Example: HO
34.0 g/mol = 2
17.0 g/mol
Therefore, the empirical formula of HO needs
to be multiplied by two in order to find the
molecular formula:
(HO)x2= H2O2