WATER of CRYSTALLISATION CALCULATIONS

hydrated and anhydrous salts

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

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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14.2b atom economy calculations  *  14.3 dilution of solutions calculations

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14.4 Water of crystallisation in a crystallised salt

• Example 14.4.1: Calculate the % of water in hydrated magnesium sulphate MgSO4.7H2O salt crystals

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

• relative formula mass = 24 + 32 + (4 x 16) + [7 x (1 + 1 + 16)] = 246

• 7 x 18 = 126 is the mass of water

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

• Example 14.4.2 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 = 24 + 32 + (4 x 16) + [7 x (1 + 1 + 16)] = 246

• 7 x 18 = 126 is the mass of water

• 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 a crucible until the mass remaining was a constant 4.00g.

• 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%

• [ Ar 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/formula mass Mr of each 'species'

• Mr CuSO4 = 64 + 32 + (4x18) = 160 and Mr 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

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

• from the calculation a few lines above Mr 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)

14.2b atom economy calculations  *  14.3 dilution of solutions calculations

Chemical & Pharmaceutical Industry Economics & Sustainability, Life Cycle Assessment, Recycling

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Above is typical periodic table used in GCSE science-chemistry specifications in doing chemical calculations, and I've 'usually' used these values in my exemplar calculations to cover most syllabuses

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