1. Empirical and Molecular Formulae
You should know this for the quiz!

2. Empirical and Molecular Formulae
Empirical Formula
Molecular Formula
If needed, change % to g.
Do a molar conversion to change from gmol
Divide each of these numbers by the smallest number
Round to the nearest whole number
Write the formula!
Determine the molar mass of the empirical formula
Divide the molar mass of the compound by the molar mass of the empirical formula
Round the ratio to the nearest whole number
Multiply each subscript in the empirical formula by the ratio number

3. Finding Percent Composition
Calculate the molar mass of the compound
Find the amount of mass due to each element in the compound
Divide each by the total molar mass
Multiply each by 100 to get your percent!
Example: H2O
Calculate molar mass: H: 2 x 1 = 2 O: 1 x 16 = 16 Molar mass 18
Amount of each H: 2/18 = = 11.11% O: 16/18 = = 88.89%