What the Percent Composition Calculator does and when to use it
Percent composition tells you what share of a compound or mixture's total mass comes from one component. It is the mass of the element or part divided by the total mass, times 100. Chemists use it to verify a compound's identity from lab data, to compare an experimental result with a theoretical formula, to work out empirical formulas, and to check the purity of a sample.
This calculator takes two masses in the same unit, the mass of the element or component and the mass of the whole sample, and returns the percentage to two decimal places. To get the full breakdown of a compound, run it once for each element and confirm that the percentages add up to about 100.
Formula and method
The formula is mass percent = (mass of component / total mass) × 100. When working from a chemical formula instead of a measurement, the component mass is the element's atomic mass times the number of atoms of it in the formula, and the total is the molar mass.
Because it is a ratio, the answer does not depend on the unit as long as both masses use the same one. Grams, kilograms and atomic mass units all give the same percent.
- m(component) the mass of the element or part you care about.
- m(total) the mass of the whole compound or sample; it must be greater than zero.
- % the mass percent, from 0 to 100 when the component is part of the total.
Worked example
Find the percent of carbon in carbon dioxide using atomic masses. One carbon atom is 12.011 and one CO₂ unit weighs 44.009 (12.011 + 2 × 15.999).
- Enter the element mass 12.011 and the total mass 44.009.
- Divide: 12.011 / 44.009 = 0.27292.
- Multiply by 100 and round to two decimals: 27.29%.
The calculator returns 27.29%. Oxygen accounts for the rest: 31.998 / 44.009 = 72.71%, and the two add to 100%. For a second check, hydrogen in water is 2.016 out of 18.015, which the calculator gives as 11.19%.
Common mistakes and how to interpret the result
- Using different units for the two masses. If one value is in grams and the other in milligrams, the percentage will be off by a factor of 1000.
- Forgetting the atom count. When using atomic masses, multiply by the subscript, for example 2 × 1.008 for the hydrogen in water.
- Entering the total as the mass of the remaining components. The total must include the component itself, otherwise the percent can exceed 100.
- Rounding too early. Keep extra digits in intermediate values and round only the final percent, especially when the percentages must sum to 100.
Related calculators
- Molar Mass Calculator — get the total mass of a formula.
- Empirical Formula Calculator — turn percentages into a formula.
- Percent Yield Calculator — compare actual and theoretical yield.
- Molecular Formula Calculator — scale an empirical formula up.