C5 Determine the mass of water present in hydrated crystals

what is water of crystallisation

water that is chemically bounded into the crystal structure/ can be removed by heating to a constant mass

Apparatus and chemicals

• Hydrated iron(II) sulfate, FeSO₄.xH₂O (1.30 g – 1.50 g)

• Spatula

• Weighing bottle

• Bunsen, tripod and pipe clay triangle

• Heat-proof mat

• Crucible

• Tongs

• Electronic balance

method

1. Weigh a crucible, record this mass value in your results table.

2. Add between 1.30 g and 1.50 g of hydrated iron(II) sulfate crystals, FeSO₄·xH₂O. Reweigh the crucible, and record the new mass in the results table.

3. Place the crucible containing the hydrated iron(II) sulfate crystals on the pipe clay triangle and gently heat for two minutes. You should avoid the formation of brown iron(III) oxide if possible.

4. Allow to cool and weigh the crucible and its contents, then record the mass.

5. Reheat the crucible and its contents and reweigh, record the new mass.

6. Continue reheating and reweighing until constant mass is observed. This process is known as ‘heating to constant mass’ and ensures that all the water of crystallisation has been removed from the hydrated crystals.