Paper 3 Advanced Practical Skills 1
9701/31 · October/November 2025
Question 2 — Thermal decomposition of hydrated zinc sulfate
Heat FA 4 in a crucible to constant mass, then use the mass lost as water of crystallisation to find y in ZnSO₄·yH₂O.
RUN IT ON THE BENCH
Your readings feed straight into the answer table below as you record them.
ON YOUR BENCH — FROM THE CONFIDENTIAL INSTRUCTIONS
Question 2 — solid
- FA 4(3.20 ± 0.20 g)
Question 2 — apparatus
- crucible with lid(1)
- crucible tongs(1)
- pipeclay triangle(1)
- tripod(1)
- Bunsen burner and means to light it(1)
- heat-proof mat(1)
- spatula(1)
- stop-clock(1)
- balance(shared, 1 per 8–12 candidates)
- wash bottle of distilled water(1)
THE BENCH — CRUCIBLE + LID, FA 4, BALANCE, TRIPOD AND BUNSEN
Why weigh the crucible with its lid on, every time?
The lid is part of the fixed mass you subtract away. As long as it's included in all four weighings, it cancels out of both subtractions — mass of FA 4 used and mass of residue. Leaving it off even once would throw off every mass after that point.
Why heat strongly, then only briefly the second time?
The first heating has to drive off essentially all the water of crystallisation, so it runs long and hot. The second heating is just a check — a short blast to confirm the mass has genuinely stopped falling. If it had dropped by more than a small amount, you'd heat again.
What should the two masses tell you?
For ZnSO₄·7H₂O, Mr = 161.5 + 7×18 = 287.5. So mass of FA 4 : mass of residue should sit close to 287.5 ÷ 161.5 ≈ 1.78. If your ratio is a long way from that, suspect a spitting loss during heating (losing solid out of the crucible would make the residue mass too low) or an incomplete first heating.
YOUR TASKS · 0/5
- ✓Weigh the empty crucible + lid
- ✓Add FA 4 and weigh crucible + lid + FA 4
- ✓Heat strongly, cool, and reweigh
- ✓Heat again briefly and reweigh — confirm constant mass
- ✓Calculate y in ZnSO₄·yH₂O
TABLE 2(a) — YOUR MASSES
| mass of crucible + lid /g | — |
| mass of crucible + lid + FA 4 /g | — |
| mass of crucible + lid + residue, 1st heating /g | — |
| mass of crucible + lid + residue, 2nd heating /g | — |
| mass of FA 4 used /g | — |
| mass of residue /g | — |
YOUR ANSWERS
Weigh the crucible with lid, then with FA 4 added, then heat and reweigh after cooling — twice, to constant mass. Record all masses and calculate the mass of FA 4 used and the mass of residue obtained.
| Thermal decomposition of FA 4 | |
|---|---|
| mass of crucible + lid /g | |
| mass of crucible + lid + FA 4 /g | |
| mass of crucible + lid + residue after first heating /g | |
| mass of crucible + lid + residue after second heating /g | |
| mass of FA 4 used /g | |
| mass of residue /g |
Calculate the amount, in mol, of anhydrous zinc sulfate residue formed in the decomposition of FA 4, and the amount, in mol, of water of crystallisation lost.
Calculate the value of y in the formula ZnSO₄·yH₂O. Show your working.
A student suggests using this thermal decomposition method to investigate the number of moles of water of crystallisation in hydrated ethanedioic acid. The teacher says that this method is unsuitable. Suggest why this method is unsuitable.