All calculations
Relative formula mass
Quantitative chemistry
AQA 8462
ORIGINAL
Mr = sum of every Ar in the formula
One missed subscript changes every later answer, so expand the formula before adding.
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
2 marks
Calculate the relative formula mass, Mr, of Al2(SO4)3. (Ar: Al = 27, S = 32, O = 16)
Do the calculation on paper first — then mark it.
Where the marks get lost
- Counting only four oxygen atoms instead of twelve.
- Adding the small formula numbers instead of multiplying each Ar.
Exam tip: Write the number of each atom underneath the formula before using the periodic table.
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.