Percent composition
Percent composition tells what fraction of a compound’s mass each element contributes.
percent of element = (mass of that element in 1 mol ÷ molar mass of compound) × 100
The percentages of all the elements add up to 100.
Worked example: water, H2O
Molar mass = 2(1.008) + 15.999 = 18.015 g/mol
| Element | Mass in 1 mol | Percent |
|---|---|---|
| H | 2.016 g | 2.016 ÷ 18.015 × 100 = 11.19% |
| O | 15.999 g | 15.999 ÷ 18.015 × 100 = 88.81% |
Water is 11.19% hydrogen and 88.81% oxygen by mass, even though it has twice as many hydrogen atoms. Oxygen atoms are much heavier.
Common mistake
Using the number of atoms instead of mass. Water is two-thirds hydrogen by atom count but only about 11% hydrogen by mass.