Solving Empirical Formulas and
Molecular Formulas
Def: A molecular formula represents the type and number of all atoms in a molecule.
Fre Previw
Def: An empirical formula is the simplest whole number ratio of elements in a
molecule. It may or may not be identical to the molecular formula.
Example: Methane has the molecular formula CH4, which is also its empirical formula.
Butane has the molecular formula C4H10, but its empirical formula is C2H5 (2:5 is the
lowest whole-number ratio)
How to solve for the empirical formula from percent composition data
Step1. Base the determination on a 100 gram sample
Step 2. Divide each mass percentage by the molar mass of the element to
determine the number of moles of each element in the 100 gram sample.
Step 3. Divide each molar amount by whichever amount is the smallest
(to get the molar ratio)
Step 4. If some results from Step 3 are far away from integers, multiply
through by a common factor that converts all molar amounts to
integers or near-integers.
Step 5. Round each molar number to the nearest integer
1. What is the empirical formula of a sample whose mass percent composition is:
40.0% C, 6.7% H, 53.3% O?