Percent Composition Calculator

This calculator finds the percent composition by mass of an element within a compound, telling you what fraction of the compound's mass comes from that element. Percent composition is a basic and useful idea in chemistry: it breaks down a compound by the mass contribution of each of its elements. For any element in a compound, the mass percent is the total mass of that element in one mole of the compound, the number of its atoms times its atomic mass, divided by the molar mass of the whole compound, expressed as a percentage. This tells you, for example, that water is about 89 percent oxygen and 11 percent hydrogen by mass, even though it has twice as many hydrogen atoms, because oxygen atoms are so much heavier. Percent composition is used to work out how much of an element a material contains, to check the purity and identity of a compound, to calculate nutrient content in fertilisers, and as a step toward finding empirical formulas. This tool makes it straightforward. You enter the atomic mass of the element, the number of atoms of that element in the formula, and the molar mass of the whole compound, and the calculator returns the percent by mass of that element, along with its mass contribution per mole. The results update as you type, so you can work through each element in turn. Use it for chemistry homework, for analysing a compound, or for any composition calculation. To find the percentage of a different element in the same compound, simply change the atomic mass and atom count while keeping the compound's molar mass the same. The percentages of all the elements in a compound add up to one hundred.

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Mass percent = (atomic mass x number of atoms / molar mass of compound) x 100. The percentages of all elements in a compound sum to 100.