Hydrated Salts | AQA A Level Chemistry Revision Notes 2017

Water of Crystallisation

Water of crystallisation is when some compounds can form crystals which have water as part of their structure
A compound that contains water of crystallisation is called a hydrated compound
The water of crystallisation is separated from the main formula by a dot when writing the chemical formula of hydrated compounds
- E.g. hydrated copper(II) sulfate is CuSO₄∙5H₂O
A compound which doesn’t contain water of crystallisation is called an anhydrous compound
- E.g. anhydrous copper(II) sulfate is CuSO₄
A compound can be hydrated to different degrees
- E.g. cobalt(II) chloride can be hydrated by six or two water molecules
- CoCl₂ ∙6H₂O or CoCl₂ ∙2H₂O

The conversion of anhydrous compounds to hydrated compounds is reversible by heating the hydrated salt:

Hydrated: CuSO₄•5H₂O ⇌ CuSO₄ + 5H₂O :Anhydrous

The degree of hydration can be calculated from experimental results:
- The mass of the hydrated salt must be measured before heating
- The salt is then heated until it reaches a constant mass
- The two mass values can be used to calculate the number of moles of water in the hydrated salt - known as the water of crystallisation

Calculating water of crystallisation

11.25 g of hydrated copper sulfate, CuSO₄.xH₂O, is heated until a constant mass of 7.19 g.

Calculate the formula of the hydrated copper(II) sulfate.

A_r (Cu) = 63.5 A_r (S) = 3. A_r (O) = 16 A_r (H) = 1

Answer:

A water of crystallisation calculation can be completed in a similar fashion to an empirical formula calculation

Instead of elements, you start with the salt and water
Instead of dividing by atomic masses, you divide by molecular / formula masses
The rest of the calculation works the same way as the empirical formula calculation