1. Calculating Molecular Formulas
Chapter 7-3

2. Empirical formula vs Molecular formula
Empirical Formula: a formula which has been reduced to the lowest terms.
Molecular formula: a formula of a compound in which the subscripts give the actual number of each element in the formula

3. Molecular Formulas Empirical formula vs Molecular formula
How to calculate molecular formulas
Ethyne and benzene have the same empirical formula CH but have a different molecular formula
Ethyne = C2H Benzene = C6H6
These two compounds have different molar masses.
The molecular formula can be determine if you know the empirical formula and the molar mass of the compound.

4. Here are four formulas being used as examples:
Molecular Formula
Empirical Formula
H2O
C2H4O2
CH2O
C6H12O6
Notice two things:
1. The molecular formula and the empirical formula can be identical.
2. You scale up from the empirical formula to the molecular formula by a whole number factor.

5. Sample Problem
Calculate the molecular formula of a compound whose molar mass is g/mol and the empirical formula is CH4N.
Calculate the molar mass of the empirical formula.
C = 1 x 12 = 12
H = 4 x 1 = 4
N = 1 x 14 = 14
Molar mass CH4N = 30 g/mol
Steps to calculate molecular formula

6. Find the ratio using molar masses
Molar mass compound ( given)
Ratio =
Molar mass empirical formula
60 g/mol
Ratio = = 2
30 g/mol

7. Determine Molecular Formula
Empirical formula x Ratio
CH4N x = C2H8N2
Molecular Formula