WATER of CRYSTALLISATION CALCULATIONS

hydrated and anhydrous salts (re-edit)

Doc Brown's Chemistry - GCSE/IGCSE/GCE (basic A level) O Level  Online Chemical Calculations

14. Other GCSE chemical calculations - WATER OF CRYSTALLISATION - method of determining and calculations

Keywords: Quantitative chemistry calculations How to determine the water of crystallisation in a salt like compound, hence determine the full formula of the hydrated salt. How to calculate the % water in a hydrated salt i.e. the percentage of water of crystallisation in a salt. How to deduce the number of molecules of water of crystallisation in a salt - fully worked out example calculations of water of crystallisation. Online practice exam chemistry CALCULATIONS and solved problems for KS4 Science GCSE/IGCSE CHEMISTRY and basic starter chemical calculations for A level AS/A2/IB courses. These revision notes and practice questions on how to do water of crystallisation chemical calculations and worked examples should prove useful for the new AQA, Edexcel and OCR GCSE (9–1) chemistry science courses.

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14.4 Water of crystallisation in a crystallised salt

• 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 from the aqueous solution,

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

• Example 14.4.1: Reminders on calculating formula mass e.g. with MgSO₄.7H₂O

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

  • You need to add together the formula mass of MgSO₄ plus the relative mass of seven water molecules.

  • 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

• Example 14.4.2 How to calculate the theoretical % of water in a hydrated salt

  • eg magnesium sulphate MgSO₄.7H₂O 'hydrated' salt crystals

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

  • Relative formula mass of crystals = 24 + 32 + (4 x 16) + {7 x (1 + 1 + 16)} = 246

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

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

• Example 14.4.3 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 and formula mass of the salt are calculated as follows:

    • Suppose 6.25g of blue hydrated copper(II) sulphate, CuSO₄.xH₂O, (x unknown) was gently heated in an evaporating dish until the mass of dish plus salt remaining was a constant leaving 4.00g of the anhydrous (dehydrated) salt.

      • (I'm just ignoring the actual masses of the evaporating dish before and after heating, but that maths is pretty simple!)

    • When the mass on subsequent weighings stays constant, you know all the water of crystallisation has driven off by the heat.

    • 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 values: Cu=64, S=32, O=16, H=1 ]

    • The mass ratio of CuSO₄ : H₂O is 4.00 : 2.25 (or 64% : 36%, doesn't matter which mass ratio you use)

    • To convert from mass ratio to mole ratio, you divide by the masses molecular /f formula mass M_r of each 'species'

    • M_r CuSO₄ = 64 + 32 + (4x18) = 160

    • and M_r 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

    • (divide through by 0.025 to get the simplest integer ratio)

    • so the formula of the hydrated salt is CuSO₄.5H₂O

    • The formula mass M_r can then be calculated as follows:

      • from the calculation a few lines above M_r CuSO₄.5H₂O = 160 + (5 x 18) = 250

• There are some More sophisticated A Level problems involving titrations to determine the water of crystallisation in a salt like compound (See Q30)

• Advanced level pre-university chemistry notes on the structure of hydrated salts

Practice questions involving water of crystallization

You must be able to do calculations involving moles and mole ratios!

Q1 The formula for the blue hydrated crystals of copper(II) sulfate is CuSO₄.5H₂O.

Relative atomic masses: Cu = 63.5   S = 32.0   O = 16.0   H = 1.00

Calculate the % water of crystallisation in the crystals (to 3 sf).

Worked out ANSWERS to the water of crystallisation questions

Q2 The hydrated salt of cobalt(II) chloride has the formula CoCl₂.xH₂O.

From a heating-dehydration experiment it was found that the hydrated salt lost the equivalent of 21.7% water.

Relative atomic masses: Co = 59.0   Cl = 35.5   O = 16.0   H = 1.00

Deduce the value of x and hence the formula of hydrated cobalt(II) chloride.

Worked out ANSWERS to the water of crystallisation questions

Q3 An evaporating dish weighed 30.55 g. Pale green crystals of hydrated iron(II) sulfate (FeSO₄.xH₂O) were added until the dish and contents weighed 40.75 g. After gentle heating to constant weight, the dish and anhydrous salt weighed 36.12 g.

Relative atomic masses: Fe = 55.8   S = 32.0   O = 16.0   H = 1.00

Deduce the value of x and hence the formula of the hydrated iron(II) sulfate salt.

Worked out ANSWERS to the water of crystallisation questions

Q4 Crystals of Glauber's, a hydrated form of the salt sodium sulfate, has the formula Na₂SO₄.xH₂O.

3.578 g of the salt was heated to dehydrate it, leaving a residue of 1.578 g

Deduce the formula of Glauber's salt.

Worked out ANSWERS to the water of crystallisation questions

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