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Percentage by Mass

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Work out what percentage of a compound’s mass comes from one element, or from water of crystallisation. Every question uses the rules below, and the worked solution tells you which rule each step needs.

The rules
  1. Work out the Mr of the whole compound first, using the relative atomic masses (Ar).
  2. Mass of the element = number of atoms × Ar. Count every atom of that element in the formula, including any that appear twice, like the two N in NH4NO3.
  3. Percentage by mass = mass of the element ÷ Mr × 100. Part on top, whole underneath.
  4. A small number after a bracket multiplies everything inside it. (NH4)2SO4 has 2 N and 8 H.
  5. In a hydrated salt, the big number multiplies the whole water molecule. CuSO4·5H2O contains 5 × 18 = 90 of water, and the Mr includes it.
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Worked solution

    Relative atomic masses are the values on the AQA GCSE periodic table, for example H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, Al = 27, S = 32, Cl = 35.5, Ca = 40, Fe = 56, Cu = 63.5, Zn = 65.

    Answers within 1% are marked correct, so give at least 3 significant figures.

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