Empirical and molecular formula
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.
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.
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)
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.
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.