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)
[4 marks]
Total for this question: 4
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.
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 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.
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