Percentage composition by mass

Percentage composition tells you what share of a compound's mass comes from each element. It is found from the formula alone.

% of an element = (atoms of it × its atomic weight) ÷ molar mass × 100

Example 1: water

Water, H₂O, has a molar mass of 18.015. Hydrogen supplies 2 × 1.0080 = 2.016. Oxygen supplies 15.999.

• Hydrogen: 2.016 ÷ 18.015 × 100 = 11.19%
• Oxygen: 15.999 ÷ 18.015 × 100 = 88.81%

Although water has two hydrogen atoms for every oxygen atom, oxygen makes up almost nine tenths of the mass, because each oxygen atom is about sixteen times as heavy as a hydrogen atom. The percentages must add up to 100. That is your check.

Example 2: nitrogen in fertilisers

Farmers buy nitrogen. Which holds more of it per kilogram, urea or ammonium sulphate?

Urea, CO(NH₂)₂, has 2 nitrogen atoms and a molar mass of 60.06. Nitrogen: 2 × 14.007 ÷ 60.06 × 100 = 46.6%.

Ammonium sulphate, (NH₄)₂SO₄, also has 2 nitrogen atoms but a molar mass of 132.13. Nitrogen: 21.2%.

Urea carries more than twice as much nitrogen for the same mass, one reason it is the most used nitrogen fertiliser.

Example 3: metal in an ore

Haematite is mainly iron(III) oxide, Fe₂O₃. Iron: 2 × 55.845 ÷ 159.69 × 100 = 69.9%. So a tonne of pure haematite can give at most about 699 kg of iron. Real ore contains rock as well, so the yield is lower.

Water in a hydrate

The same idea gives the percentage of water of crystallisation. In washing soda, Na₂CO₃·10H₂O, the ten waters have a mass of 180.15 out of 286.14: 63.0% of the crystal is water.

Mass of an element in a given sample

Multiply the sample mass by the percentage. How much calcium is in a 500 mg tablet of calcium carbonate? Calcium is 40.04% of CaCO₃, so the tablet holds 500 × 0.4004 = 200 mg of calcium.

Going backwards

If you are given the percentages and asked for the formula, you are finding an empirical formula. That method is on the empirical and molecular formula page.

The calculator shows the percentage of every element for any formula you type, and every compound page has the full breakdown.

Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.

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Empirical and molecular formula All guides