Percent Composition Calculator
Percent composition tells you what share of a compound's mass comes from each element. Chemistry classes and lab work use it to check a formula. Enter how many atoms of each element the compound has, and their atomic masses.
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How it works
Multiply the atomic mass of an element by its number of atoms to get that element's total mass in one mole. Add the totals to get the molar mass of the compound. Each element's share is its total divided by the molar mass, times 100.
Atomic masses are listed on any periodic table. Round them the same way for both elements, and the shares will add up to close to 100 percent.
Worked example
In a compound with a molar mass of 18.015 grams per mole, element 1 makes up 11.2 percent of the mass and element 2 makes up 88.8 percent.
Questions
Do the two shares always add up to 100?
Only when the compound has just these two elements. A third element takes up the rest of the mass, which this page does not show.
Where do I get atomic masses?
A periodic table lists them. They are the average masses of an element's atoms, usually given to three or more decimal places.
Can I use this for a formula with more elements?
This page takes two elements at a time. For more, add each extra element's total to the molar mass and divide it the same way.
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