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A LevelChemistryCIERevision Notes1. Physical Chemistry1.2 Atoms, Molecules & Stoichiometry1.2.6 Water of Crystallisation

Water of Crystallisation (CIE A Level Chemistry)

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Water of Crystallisation

  • Water of crystallisation is when some compounds can form crystals which have water as part of their structure
  • A compound that contains water of crystallisation is called a hydrated compound
  • The water of crystallisation is separated from the main formula by a dot when writing the chemical formula of hydrated compounds
    • E.g. hydrated copper(II) sulfate is CuSO4∙5H2O

  • A compound which doesn’t contain water of crystallisation is called an anhydrous compound
    • E.g. anhydrous copper(II) sulfate is CuSO4

  • A compound can be hydrated to different degrees
    • E.g. cobalt(II) chloride can be hydrated by six or two water molecules
    • CoCl2 ∙6H2O or CoCl2 ∙2H2O

  • The conversion of anhydrous compounds to hydrated compounds is reversible by heating the hydrated salt:
Hydrated:        CuSO4•5H2O ⇌ CuSO4 + 5H2O        :Anhydrous

  • The degree of hydration can be calculated from experimental results:
    • The mass of the hydrated salt must be measured before heating
    • The salt is then heated until it reaches a constant mass
    • The two mass values can be used to calculate the number of moles of water in the hydrated salt - known as the water of crystallisation

Worked example

Calculating water of crystallisation

11.25 g of hydrated copper sulfate, CuSO4.xH2O, is heated until a constant mass of 7.19 g.

Calculate the formula of the hydrated copper(II) sulfate.

Ar (Cu) = 63.5      Ar (S) = 3.      Ar (O) = 16     Ar (H) = 1

Answer:

1. Salt and water CuSO4 H2O
2. Value 7.19 11.25 - 7.19
= 4.06
3. Mr  63.5 + 32 + (16 x 4)
= 159.5		 (1 x 2) + 16
= 18
4. Moles = mass over M subscript r fraction numerator 7.19 over denominator 159.5 end fraction = 0.045 fraction numerator 4.06 over denominator 18 end fraction = 0.226
5. Salt : water ratio fraction numerator 0.045 over denominator 0.045 end fraction = 1 fraction numerator 0.226 over denominator 0.045 end fraction = 5
6. Formula of hydrated salt The formula is CuSO4•5H2O

Examiner Tip

A water of crystallisation calculation can be completed in a similar fashion to an empirical formula calculation
  • Instead of elements, you start with the salt and water
  • Instead of dividing by atomic masses, you divide by molecular / formula masses
  • The rest of the calculation works the same way as the empirical formula calculation

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    Richard

    Author: Richard

    Expertise: Chemistry

    Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.