8. Empirical Formulae and Water of Crystallisation
If you leave a saturated solution of copper(II) sulphate in a crystallising dish, some of the
water evaporates and beautiful blue diamond-shaped crystals form. These hydrated crystals
contain a fixed number of trapped water molecules. This is known as water of
crystallisation.In the case of hydrated copper(II)
sulphate, there are exactly five moles of water for
every mole of copper(II) sulphate, so the formula is
written as CuSO4⋅5H2O. A crystal that contains water of
crystallisation is described as hydrated. If there is no
water of crystallisation, the salt is anhydrous.
CuSO4⋅5H2O(s) → CuSO4(s) + 5H2O(l)
anhydrous copper(II)
hydrated copper(II)
sulphate (white
sulphate (blue crystals)
powder)
Example
40g of hydrated copper(II) sulphate crystals were heated, driving off the water of
crystallisation, leaving 25.6g of anhydrous copper(II) sulphate. Show that this is consistent
with hydrated copper(II) sulphate having the formula CuSO4⋅5H2O.
These headings don’t always have to be elements.
Here we are working out the ratio of moles of two
compounds; copper(II) sulphate and water.
CuSO4 H2O
mass 25.6g 14.4g
25.6 14.4
moles = 0.16 mol = 0.8 mol
160 18
ratio of moles 1 : 5
empirical formula CuSO4⋅5H2O
1. A student found that a sample of 30.5g to 26.0g. Work out the formula of
hydrated cobalt(II) chloride contained the hydrated salt.
5.2g of anhydrous cobalt chloride and
4.32g of water. Calculate the formula of 4. Hydrated sodium carbonate,
hydrated cobalt(II) chloride. Na2CO3⋅xH2O was found to contain 62.9%
water by mass. Calculate the value of x.
2. 13.9g of hydrated iron(II) sulphate
Answers
crystals were heated, driving off the water 5. Potash alum has the 1. CoCl2⋅6H2O
of crystallisation, leaving 7.6g of formula 2. FeSO4⋅7H2O
anhydrous iron(II) sulphate. Calculate the K2SO4⋅Al2(SO4)3⋅yH2O. 3. BaCl2⋅2H2O
4. x = 10
formula of hydrated iron(II) sulphate. Calculate y, the number 5. y = 24
of moles of water of
3. A sample of hydrated barium chloride crystallisation, given that the hydrated
was heated to drive off water of salt contains 45.57% water by mass.
crystallisation. Its mass decreased from