A compound contains 40.0% C, 6.7% H and 53.3% O by mass. Its Mr is 180. Determine its empirical and molecular formulae. (Ar: C = 12, H = 1, O = 16)
[4 marks]
Total for this question: 4
All calculations
Empirical and molecular formula
Amount of substance
AQA 7405
ORIGINAL
empirical ratio = mass / Ar; molecular factor = Mr / empirical-formula mass
Percentages become masses out of 100 g; masses become moles; only then can you simplify the ratio.
Board and spec code confirmed against AQA 7405 · registry checked 2026-07-11How 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 percentage values as the atom ratio without dividing by Ar.
- Multiplying by Mr instead of dividing Mr by the empirical-formula mass.
Exam tip: Keep unrounded mole values until the ratio is clear, then check the molecular formula has the stated Mr.
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