Percent Composition Calculator (Mass Percent by Element)
Percent composition is each element's share of the molar mass. In Fe2O3, iron contributes 2 × 55.845 = 111.69 of the 159.687 g/mol, so it is 111.69 ÷ 159.687 × 100 = 69.94% iron and 30.06% oxygen.
Add a mass to see how many grams of each element it contains.
Largest share in Fe₂O₃
Fe 69.94%
Molar mass
159.687 g/mol
| Element | Atoms | Mass % |
|---|---|---|
| FeIron | 2 | 69.94% |
| OOxygen | 3 | 30.06% |
Show the math
- 1
Molar mass of Fe₂O₃
2 × 55.845 + 3 × 15.999 = 159.687 g/mol
Add up the atomic masses of every atom in the formula.
= 159.687 g/mol
Read the steps as text
- Molar mass of Fe₂O₃. 2 × 55.845 + 3 × 15.999 = 159.687 g/mol Add up the atomic masses of every atom in the formula.
- Iron (Fe). 111.69 ÷ 159.687 × 100 = 69.94% Divide the element's share of the molar mass (2 × 55.845) by the whole, then multiply by 100.
- Oxygen (O). 47.997 ÷ 159.687 × 100 = 30.06%
How percent composition is calculated
First the calculator works out the compound's molar mass by adding up the atomic masses of every atom in the formula. Then, for each element, it takes that element's contribution (number of atoms × atomic mass), divides it by the molar mass and multiplies by 100.
Water is a simple example. Its molar mass is 18.015 g/mol, of which the two hydrogens contribute 2.016. So water is 2.016 ÷ 18.015 × 100 = 11.19% hydrogen by mass, and the remaining 88.81% is oxygen. The percentages always add up to 100%, give or take rounding.
Finding the mass of an element in a sample
Because the percentages don't depend on how much of the compound you have, they also tell you how much of each element is in any sample. Enter a sample mass and the calculator multiplies it by each percentage: 50 g of Fe2O3 contains 50 × 69.94% = 34.97 g of iron.
This is the same idea used to compare fertilizers by nitrogen content, or ores by how much metal they yield. To go the other way, from measured percentages back to a formula, use the empirical formula calculator.
Frequently asked questions
- What is the formula for percent composition?
- Mass percent of an element = (number of atoms × atomic mass ÷ molar mass of the compound) × 100.
- Is percent composition the same as mass percent?
- Yes. Percent composition by mass, mass percent and percent by mass all mean each element's share of the compound's total mass.
- Why don't my percentages add up to exactly 100%?
- Each one is rounded to two decimal places, so the total can be off by 0.01% or so. The unrounded values always add up to exactly 100%.
- Does the amount of compound change the percent composition?
- No. A gram or a ton of water is 11.19% hydrogen by mass. Only the grams of each element change with the sample size.