Determine the mass of water present in hydrated crystals

A solid which is hydrated contains ‘water of crystallisation’. This is water which is ‘chemically combined’ in the crystal structure. If we heat a hydrated solid gently, the water will be released and the solid will lose mass.

In this experiment you will heat hydrated iron(II) sulfate crystals. By carefully heating the crystals and recording mass measurements, we can calculate the mass of water in the supplied crystals.

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

  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.