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Fertilizer bags print three numbers on the front. Iron supplements list “elemental iron” separately from the compound they contain. Mining companies price ore by how much metal it holds. All three are really asking the same question: what fraction of this substance’s mass comes from one particular element?
That fraction, written as a percentage, is the substance’s percent composition.
The formula
For any element in a compound:
mass % of element = (number of atoms × atomic mass) ÷ molar mass of compound × 100
The numerator is how much mass that element contributes to one mole of the compound. The denominator is the mass of the whole mole. Everything else is just converting a fraction into a percentage.
Worked example: water
H₂O has a molar mass of 18.015 g/mol.
- Hydrogen: 2 × 1.008 = 2.016 → 2.016 ÷ 18.015 × 100 = 11.19%
- Oxygen: 1 × 15.999 = 15.999 → 15.999 ÷ 18.015 × 100 = 88.81%
Check: 11.19 + 88.81 = 100.00. The percentages for any compound always add up to 100 (give or take rounding), and that makes a handy built-in check on your arithmetic.
There’s something worth noticing here. Two out of every three atoms in water are hydrogen, yet hydrogen is only about 11% of the mass. Oxygen atoms are roughly sixteen times heavier, so they dominate the weight. Percent by mass and percent by number of atoms are completely different things, and mixing them up is a classic error.
Worked example: glucose
C₆H₁₂O₆, molar mass 180.16 g/mol.
| Element | Contribution (g/mol) | Mass % |
|---|---|---|
| Carbon | 6 × 12.011 = 72.07 | 40.00% |
| Hydrogen | 12 × 1.008 = 12.10 | 6.71% |
| Oxygen | 6 × 15.999 = 96.00 | 53.29% |
Glucose is more than half oxygen by mass. Those three numbers — 40.00, 6.71 and 53.29 — are also exactly what a lab analysis of pure glucose would report, which leads to the most important use of percent composition.
Worked example: a compound with parentheses
Ammonium sulfate, (NH₄)₂SO₄, is a common nitrogen fertilizer. How much of it is actually nitrogen?
Count the atoms carefully: the subscript 2 outside the parentheses doubles everything inside, so there are 2 N and 8 H, plus 1 S and 4 O.
- Molar mass: 2(14.007) + 8(1.008) + 32.06 + 4(15.999) = 132.14 g/mol
- Nitrogen: 28.014 ÷ 132.14 × 100 = 21.2%
So a 50 kg bag delivers about 10.6 kg of nitrogen. Compare urea, CO(NH₂)₂: its two nitrogens weigh 28.014 out of a molar mass of 60.06, which is 46.6% nitrogen. Per kilogram, urea delivers more than twice as much, which is one reason it’s the most widely used nitrogen fertilizer in the world.
Going backwards: from percentages to a formula
Percent composition is also the starting point for identifying an unknown compound. A combustion analyzer can measure what percentage of a sample is carbon, hydrogen and so on. From those numbers you can work back to the simplest whole-number ratio of atoms — the empirical formula.
The trick is to imagine a 100 g sample, so each percentage becomes a mass in grams, then convert each mass to moles and compare. We walk through the full method in empirical vs. molecular formulas.
Grams of an element in a sample
Once you know the mass percent, finding how much of an element a real sample contains is one multiplication.
How many grams of iron are in a 325 mg tablet of iron(II) sulfate, FeSO₄?
- Molar mass of FeSO₄: 55.845 + 32.06 + 4(15.999) = 151.90 g/mol
- Iron: 55.845 ÷ 151.90 × 100 = 36.76%
- Iron in the tablet: 325 mg × 0.3676 = 119 mg
(Supplements are often sold as the heptahydrate, FeSO₄·7H₂O, whose extra water drops the iron content to about 20%. That’s exactly why labels list “elemental iron” separately — the salt’s mass alone doesn’t tell you.)
Common mistakes
- Using atom counts instead of masses. Water is not “66.7% hydrogen” by mass.
- Forgetting the parentheses multiplier, as in (NH₄)₂SO₄.
- Dropping the water in a hydrate. CuSO₄·5H₂O is 25.45% copper; anhydrous CuSO₄ is 39.82% copper. Both are correct for their own substance.
- Rounding each percentage too early. Work with unrounded contributions and round at the end, or your total may land at 99.8% or 100.3%.
Quick answers
Is percent composition the same as mass percent? Yes, when people say “percent composition” without qualification they almost always mean percent by mass.
Does percent composition depend on the size of the sample? No. A gram of pure water and a tonne of pure water are both 11.19% hydrogen. It’s a property of the compound, not the sample.
Why don’t my lab percentages add up to 100%? Either an element wasn’t measured (oxygen often isn’t — it’s usually found by subtracting the others from 100), or the sample wasn’t pure, or there’s measurement error.
Try it
The percent composition calculator gives the mass percent of every element for any formula — parentheses and hydrates included — and, if you enter a sample mass, the grams of each element it contains.
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