Percent Composition Calculator
Enter a chemical formula to find the percent by mass of each element. The tool computes the molar mass, then divides each element's mass contribution by the total.
How to calculate percent composition
- Enter a chemical formula, for example H2O, CO2 or Ca(OH)2.
- The tool finds the molar mass and each element's mass contribution.
- Read the percent by mass of every element, which sums to about 100 percent.
Examples
Water
H2O
H 11.19%, O 88.81%
Carbon dioxide
CO2
C 27.29%, O 72.71%
Frequently asked questions
What is percent composition?
Percent composition is the percentage by mass of each element in a compound. For one element it is its total mass in the formula divided by the molar mass, times 100.
How is it calculated?
First find the molar mass of the compound. Then for each element, multiply its atomic mass by how many atoms appear, divide by the molar mass, and multiply by 100.
Why do the percentages add up to about 100?
Every atom's mass is part of the whole, so the element percentages must total 100 percent. Tiny rounding in the last decimal place can make it 99.99 or 100.01.
What is the percent composition of water?
Water, H2O, is about 11.19 percent hydrogen and 88.81 percent oxygen by mass, because one oxygen atom is far heavier than the two hydrogen atoms.
Can I enter parentheses and hydrates?
Yes. Use parentheses for groups such as Ca(OH)2 and a dot for hydrates such as CuSO4.5H2O. Whitespace is ignored and unknown elements are flagged.
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