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.
Verified against AQA 8462 (2026 spec)
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
3 marks
Calculate the percentage by mass of oxygen in calcium carbonate, CaCO3. (Ar: Ca = 40, C = 12, O = 16)
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.
Calculations are the most trainable marks in chemistry
They come up every paper and reward a clean method. Send me one you keep dropping marks on — your first lesson is free.