All calculations
Water of crystallisation
Amount of substance
AQA 7405
ORIGINAL
x = moles water / moles anhydrous salt
The mass lost is water; the mass left is anhydrous salt. Convert both to moles before finding x.
Verified against AQA 7405 (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
4 marks
12.3 g of MgSO4.xH2O is heated to constant mass, leaving 6.00 g of MgSO4. Determine x. (Mr: MgSO4 = 120, H2O = 18.0)
Do the calculation on paper first — then mark it.
Where the marks get lost
- Using the initial hydrate mass as the water mass.
- Dividing the salt moles by water moles, which inverts x.
Exam tip: Heating to constant mass is the evidence that all removable water has been driven off.
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.