Empirical and Molecular
Formulas
What formulas????
Empirical Formula
 Simplest/general formula for chemical compound
 Many compounds can have the SAME empirical formula
 Lowest whole number ratio of atoms in a compound
 Sometimes actual chemical formula
 Ionic compounds, some molecular compounds
Molecular Formulas
 “true” formula
 Represents the true number of atoms of each element
in a chemical compound
 Multiple of empirical formula
Molecular Formula = (empirical formula)n
C6H6 =
Calculating the Empirical
Formula
1) Assume 100g sample of chemical compound. Find the
moles of each element in 100g of compound.
1) Divide each mole value by the SMALLEST value!
1) Multiply each number by integer to get ALL whole
numbers.
Example 1:
 Vitamin C is made up of 40.9% C, 54.5%
O, and 4.58% H. What is the empirical
formula? If Vitamin C has a molecular
mass of 176g. What is the molecular
formula?
Example 2:
Adipic acid contains 49.32%C, 43.84%O, and 6.85%H by
mass. What is the empirical formula for adipic acid?
How do we calculate the
molecular formula?
 MUST know two pieces of information
 Empirical Formula or Percent Composition of a
compound.
AND
 Molecular Mass (formula mass for the entire compound)
**Divide the molecular mass by the empirical formula mass.
MM/EM = multiple
(empirical formula)multiple = molecular formula
Example 3:
 Trichloroisocyanuric acid, found in household bleaches,
has an empirical formula of OCNCl. The molar mass of
the compound is 232.41g. What is the compound’s
molecular formula?
Example 4:
 A chemical compound is found to have an empirical
formula of NH2. The molecular mass is 32.06g. What is
the molecular formula for this compound?
Practice, Practice !!!