Thermal decomposition: water of crystallisation in hydrated zinc sulfate
Heat hydrated zinc sulfate, FA 4 (ZnSO₄·yH₂O), in a crucible until its mass stops changing, then use the mass of water driven off to calculate y — the number of water molecules per formula unit.
WHAT YOU'LL LEARN
- ✓Weighing to 0.01 g and getting four consistent, usable masses from one sample
- ✓Why heating to constant mass (heat, cool, reweigh, heat again, reweigh again) is the only way to be sure a reaction has actually finished
- ✓Calculating moles from mass and Mr, then combining two mole values into a whole-number ratio
- ✓Why a crucible needs its lid on for every single weighing
ON YOUR BENCH
- □Crucible with lid, ~15 cm³ capacity
- □Crucible tongs
- □Pipeclay triangle
- □Tripod
- □Bunsen burner and means to light it
- □Heatproof mat
- □Spatula
- □Stop-clock
- □Balance, 0.01 g accuracy
- □Wash bottle of distilled water
- □FA 4 — hydrated zinc sulfate, ZnSO₄·yH₂O, about 3.20 ± 0.20 g
The protocol, step by step
The same guide is printed and waiting at your bench.
- 01
Weigh the empty crucible + lid
Place the empty crucible with its lid on the balance and record the mass to 0.01 g. The lid stays on for every weighing from here on — it's part of the fixed mass you'll subtract away twice.
- 02
Add FA 4 and weigh again
Use the spatula to transfer all of the supplied FA 4 into the crucible. Replace the lid and weigh crucible + lid + FA 4. The difference from the first mass is the mass of FA 4 you actually used.
- 03
Heat strongly
Set the crucible in the pipeclay triangle on the tripod, lid slightly ajar so steam can escape without solid spitting out, and heat strongly with the Bunsen for several minutes. Time it with the stop-clock so your heating is reproducible.
- 04
Cool and weigh
Using tongs, move the crucible to the heatproof mat and let it cool to room temperature before weighing — a hot crucible sets up convection currents that make the balance drift and read low. Record crucible + lid + residue.
- 05
Heat again, briefly
Heat the crucible again for a shorter time, then cool and reweigh. This is the check, not the main event: if the mass barely moves, all the water was already gone after the first heating.
- 06
Confirm constant mass
Compare the two post-heating masses. If they agree closely (within a couple of hundredths of a gram), you've heated to constant mass. If the second mass is still noticeably lower, heat again until two consecutive masses agree.
- 07
Calculate
Mass of FA 4 used = (crucible+lid+FA4) − (crucible+lid). Mass of residue = (crucible+lid+residue, final) − (crucible+lid). Moles of water lost = (mass FA4 used − mass residue) ÷ 18. Moles of anhydrous ZnSO₄ = mass residue ÷ 161.5. y = moles of water ÷ moles of ZnSO₄.
What you should see
- →Mass of residue should be noticeably less than mass of FA 4 used — roughly 56% of it, since anhydrous ZnSO₄ (Mr 161.5) is a much smaller fraction of ZnSO₄·7H₂O (Mr 287.5) than the water is.
- →Mass of FA 4 used : mass of residue should land close to 1.78 — the theoretical ratio for the heptahydrate, ZnSO₄·7H₂O. A long way off usually means solid spat out during heating (residue too low) or the first heating wasn't thorough enough (residue too high).
- →The second heating's mass should barely differ from the first's — that agreement is the entire point of heating to constant mass.
- →y should come out close to a small whole number — 7 for the common heptahydrate — even though the exact starting mass varies candidate to candidate.
| Mass /g | |
|---|---|
| crucible + lid | 15.42 |
| crucible + lid + FA 4 | 18.61 |
| crucible + lid + residue, 1st heating | 17.24 |
| crucible + lid + residue, 2nd heating | 17.22 |
| mass of FA 4 used | 3.19 |
| mass of residue | 1.80 |
⚠ BEFORE YOU START
- •Wear goggles throughout — a strongly heated crucible and its contents stay dangerously hot long after the flame is off.
- •Only ever move the crucible with tongs, and set it down on the heatproof mat, never straight onto the bench.
- •Point the crucible's lid gap away from yourself and others — escaping steam can carry fine solid with it.
Try it, then run it for real.
Practise the whole thing on the virtual bench, then book real lab time and do it with your own hands.