Water of Crystallisation (Cambridge International A-Level Chemistry)

Spec point — 2.3.4

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 hydrated compounds to anhydrous compounds is achieved by heating the hydrated salt

• This process is reversed by adding water, which reforms the hydrated compound:

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

Worked Example 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 / Mr	$\frac{5.59}{159.6}$ = 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

• So, the value of x is 7
• Therefore, the formula of the hydrated salt is CuSO₄•7H₂O

Exam Hint 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