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:
- 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
10.0 g of hydrated copper sulfate are heated to a constant mass of 5.59 g. Calculate the formula of the original hydrated copper sulfate.
(Mr data: CuSO4 = 159.6, H2O = 18.0)
Answer
List the components |
CuSO4 |
H2O |
Note the mass of each component |
5.59 g |
10 - 5.59 = 4.41 g |
Divide the component mass by the components Mr |
= 0.035 |
= 0.245 |
Divide by the lowest figure to obtain the ratio |
= 1 |
= 7 |
Hydrated salt formula |
CuSO4•7H2O |
Exam Tip
- 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