5. Determine the mass of water in hydrated crystals

Question 1

Determine the mass of water present in hydrated crystals

Answer 1

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
Safety considerations

Gentle heating should be carried out to reduce risk of FeSO4 decomposing, use a well-ventilated lab
Iron(II) sulfate is harmful
Wear safety goggles
Take care when heating, all apparatus will become very hot
Method

Weigh a crucible, record this mass value in your results table.
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.
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.
Allow to cool and weigh the crucible and its contents, then record the mass.
Reheat the crucible and its contents and reweigh, record the new mass.
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.