Percentage by mass of an element = (mass contribution of that element ÷ Mr of the compound) × 100.
Count all atoms of the chosen element. You are comparing masses, not the fraction of atoms.
Use these relative atomic masses, Ar: H 1; C 12; N 14; O 16; Na 23; Mg 24; Al 27; S 32; Cl 35.5; K 39; Ca 40; Fe 56; Cu 64. Use values given in your exam question.
CaCO₃ has Mr = 40 + 12 + 48 = 100.
Calcium contributes 40.
Percentage calcium = 40 ÷ 100 × 100 = 40%.
Water, H₂O, has Mr = 18. Oxygen contributes 16.
16 ÷ 18 × 100 = 88.888…% ≈ 88.9%.
Oxygen is only one of the three atoms, but it contributes most of the mass.
In MgCl₂, chlorine contributes 2 × 35.5 = 71; Mr = 95.
71 ÷ 95 × 100 = 74.7% to 3 significant figures.
Keep extra digits during working and round your final answer as the question asks. The percentages of all elements in a compound add to about 100%; small differences can come from rounding.
Count every atom of the element. Divide its total mass contribution by Mᵣ, then multiply by 100.
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