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

10.0 g of hydrated copper sulfate are heated to a constant mass of 5.59 g.

Determine the formula of the original hydrated copper sulfate, CuSO₄•xH₂O.

(M_r data: CuSO₄ = 159.6, H₂O = 18.0)

Answer:

Compounds	CuSO₄	H₂O
Mass of each compound (g)	5.59	4.41
Formula mass	159.6	18.0
Moles = mass / M_r	$\frac{5.59}{159.6}$ = 0.035	$\frac{4.41}{18.0}$ = 0.245
Ratio (divide by smallest value)	$\frac{0.035}{0.035}$ = 1	$\frac{0.245}{0.035}$ = 7

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

Water of Crystallisation (CIE A Level Chemistry)