5. Water of crystallisation in salt crystals

The explanation and calculation of water of crystallisation

(Suitable for AQA, Edexcel and OCR GCSE chemistry students)

5. Water of crystallisation calculations

• What is water of crystallisation?

  • Water of crystallization are the molecules of water that are incorporated into some salt crystals when they are crystallised out of water.

  • e.g. when blue crystals of copper(II) sulfate are crystallized out of water the actual formula of the crystals is ...

  • NOT simply CuSO₄, but on crystallisation CuSO₄.5H₂O is formed,

  • because five water molecules are associated with each 'CuSO₄'. in its 'hydrated' crystalline form.

• Solubility graphs and data are covered in section 4.

• How to calculate the theoretical % of water in a hydrated salt

  • eg magnesium sulphate MgSO₄.7H₂O salt crystals

  • Relative atomic masses: Mg = 24, S = 32, O = 16 and H = 1

  • Relative formula mass of water = (2 x 1) + 16 = 18

  • Relative formula mass of MgSO₄ = 24 + 32 + (4 x 16) = 120

  • Relative mass of seven water molecules = 7 x 18 = 126

  • Relative formula mass of crystals = MgSO₄ + (7 x H₂O) = 120 + 126 = 246

  • so % water = 126 x 100 / 246 = 51.2%

• Determination and calculation of salt formula containing 'water of crystallisation'.

  • Some salts, when crystallised from aqueous solution, incorporate water molecules into the structure. This is known as 'water of crystallisation', and the 'hydrated' form of the compound.

  • e.g. magnesium sulphate MgSO₄.7H₂O. The formula can be determined by a simple experiment (see the copper sulphate example below).

  • A known mass of the hydrated salt is gently heated in a crucible until no further water is driven off and the weight remains constant despite further heating. The mass of the anhydrous salt left is measured. The original mass of hydrated salt and the mass of the anhydrous salt residue can be worked out from the various weighings.

  • The % water of crystallisation and the formula of the salt are calculated as follows:

    • Suppose 6.25g of blue hydrated copper(II) sulphate, CuSO₄.xH₂O, (x unknown) was gently heated in a crucible until the mass remaining was 4.00g. This is the white anhydrous copper(II) sulphate.

    • The mass of anhydrous salt = 4.00g, mass of water (of crystallisation) driven off = 6.25-4.00 = 2.25g

    • The % water of crystallisation in the crystals  is 2.25 x 100 / 6.25 = 36%

    • [ A_r's Cu=64, S=32, O=16, H=1 ]

    • The mass ratio of CuSO₄ : H₂O is 4.00 : 2.25

    • To convert from mass ratio to mole ratio, you divide by the molecular mass of each 'species'

    • CuSO₄ = 64 + 32 + (4x18) = 160 and H₂O = 1+1+16 = 18

    • The mole ratio of CuSO₄ : H₂O is 4.00/160 : 2.25/18

    • which is 0.025 : 0.125 or 1 : 5, so the formula of the hydrated salt is CuSO₄.5H₂O

• All concentration calculations are covered on the on-line calculations page, especially sections 7. on molarity, 11. and 12. on molarity and acid-base (alkali) titrations, section 14.3 on dilutions.

• GCE A level advanced notes on the structure of hydrated salts

Extra Aqueous Chemistry Index:

1. Water cycle, treatment, pollution

2. Colloids - sols, foam and emulsions

3. Hard and soft water - causes and treatment

4. Gas and salt solubility in water and solubility curves

5. Calculation of water of crystallisation (this page)

 Doc Brown's Chemistry 

*