Calculate the percentage by mass of oxygen in calcium carbonate, CaCO3. (Ar: Ca = 40, C = 12, O = 16)
[3 marks]
Total for this question: 3
All calculations
Percentage by mass
Quantitative chemistry
AQA 8462
ORIGINAL
Higher tier
% by mass = (number of atoms x Ar / Mr) x 100
Count every atom of the chosen element, then compare its total Ar contribution with the whole Mr.
Board and spec code confirmed against AQA 8462 · registry checked 2026-07-17How this checking works
Work it, then mark it
Do each calculation on paper first, then reveal the mark scheme and tick the marks you actually earned — the same way you should mark past papers.
Q1
Do the calculation on paper first — then mark it.
Where the marks get lost
- Using one oxygen Ar instead of three.
- Dividing the whole Mr by the oxygen contribution.
Exam tip: The percentages of all elements in the formula should add to 100% apart from rounding.
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