C1 - Determine the mass of water in hydrated crystals

• A hydrated solid contains water of crystallisation. This water can be removed by gentle heating with a Bunsen burner.

• By heating and weighing some solid until the mass no longer decreases (heating to constant mass), the mass of water in the hydrated crystals can be determined.

A hydrated solid contains water of crystallisation.

This is water which is chemically combined to the crystal structure.

If we heat a hydrated solid gently, the water will be released and the solid will lose mass.

In this experiment, a hydrated solid is heated to remove the water of crystallisation.

By weighing the solid before and after heating, you can calculate the mass of water that present in the hydrated compound.

An example of a hydrated compound that can be used in this experiment is hydrated iron(II) sulfate:

FeSO₄·xH₂O

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

• Spatula

• Bunsen burner, tripod and pipe clay triangle

• Heat-proof mat

• Crucible

• Tongs

• Electronic balance

• Gentle heating should be carried out to reduce the risk of the iron(II) sulfate decomposing; use a well-ventilated lab.

• Iron(II) sulfate is harmful - wear safety goggles.

• Take care when heating, all apparatus will become very hot.

1. Weigh an evaporating basin, 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 evaporating basin, and record the new mass in the results table.

3. Place the evaporating basin containing the hydrated iron(II) sulfate crystals on the gauze and gently heat for two minutes.

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

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

6. Continue reheating and reweighing until the mass does not decrease any further. This process is known as ‘heating to constant mass’ and ensures that all the water of crystallisation has been removed from the hydrated crystals.

• Steam is given off from the hydrated solid as it is heated.

• There can be a colour change in the hydrated solid.

• The crystals in the hydrated solid can break down into to a powder.

Analysis of data

Mass of hydrated crystals __________________________g

Mass of anhydrous crystals __________________________g

Mass of water of crystallisation __________________________g