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

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,

  • 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 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%

    • [ 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/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, 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)

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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

OTHER CALCULATION PAGES

1. What is relative atomic mass?, relative isotopic mass and calculating relative atomic mass

2. Calculating relative formula/molecular mass of a compound or element molecule

3. Law of Conservation of Mass and simple reacting mass calculations

4. Composition by percentage mass of elements in a compound

5. Empirical formula and formula mass of a compound from reacting masses (easy start, not using moles)

6. Reacting mass ratio calculations of reactants and products from equations (NOT using moles) and brief mention of actual percent % yield and theoretical yield, atom economy and formula mass determination

   • Reacting masses, concentration of solution and volumetric titration calculations (NOT using moles)

7. Introducing moles: The connection between moles, mass and formula mass - the basis of reacting mole ratio calculations (relating reacting masses and formula mass)

8. Using moles to calculate empirical formula and deduce molecular formula of a compound/molecule (starting with reacting masses or % composition)

9. Moles and the molar volume of a gas, Avogadro's Law

10. Reacting gas volume ratios, Avogadro's Law and Gay-Lussac's Law (ratio of gaseous reactants-products)

11. Molarity, volumes and solution concentrations (and diagrams of apparatus)

12. How to do acid-alkali titration calculations, diagrams of apparatus, details of procedures

13. Electrolysis products calculations (negative cathode and positive anode products)

14. Other calculations e.g. % purity, % percentage & theoretical yield, dilution of solutions (and diagrams of apparatus), water of crystallisation, quantity of reactants required, atom economy

    • 14.1 % purity of a product 14.2a % reaction yield 14.2b atom economy 14.3 dilution of solutions

    • 14.4 water of crystallisation calculation  14.5 how much of a reactant is needed? limiting reactant

15. Energy transfers in physical/chemical changes, exothermic/endothermic reactions

16. Gas calculations involving PVT relationships, Boyle's and Charles Laws

17. Radioactivity & half-life calculations including dating materials

18. Some Advanced A Level Practical Exercises and Calculations

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