1. Percentage Composition

2. Percent Part X 100% Whole Percent by mass = Mass of element X 100%
Mass of compound

3. Percent Composition determined by analysis
Or, if know formula assume have 1 mole
Divide mass each element by formula mass (FM) of compound

4. Percent Composition of H2O
2 H’s = 2 x 1.0 g = 2.0 g
1 O = 1 x 16.0 g = 16.0 g
Molar mass = 2.0 g g = 18.0 g
% H = 2.0g x 100% = 11.11%
18.0g
% O = 16.0g x 100% = 88.89%

5. Percent Composition of C6H12O6
6 C’s = 6 x 12g = 72g
12 H’s = 12 x 1g = 12g
6 O’s = 6 x 16g = 96g
Molar mass = grams/mole
% C =
% H =
% O =
(72g/180g) X 100% = 40.00%
(12g/180g) X 100% = 6.67%
(96g/180g) X 100% = 53.33%

6. Percentage Composition
Remember have to calculate FM!
Nowhere in word problem will it tell you that info!
Sum of individual element %’s must add up to exactly 100%