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ASChemistryCIERevision Notes2. Atoms, Molecules & Stoichiometry2.3 FormulasWater of Crystallisation

Water of Crystallisation (CIE AS 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

10.0 g of hydrated copper sulfate are heated to a constant mass of 5.59 g.

Determine the formula of the original hydrated copper sulfate, CuSO4•xH2O.

(Mr data: CuSO4 = 159.6, H2O = 18.0) 

Answer:

Compounds CuSO4 H2O
Mass of each compound
(g)		 5.59 4.41
Formula mass 159.6 18.0
Moles = mass / Mr fraction numerator 5.59 over denominator 159.6 end fraction = 0.035 begin mathsize 14px style fraction numerator 4.41 over denominator 18.0 end fraction end style = 0.245
Ratio (divide by smallest value) begin mathsize 14px style fraction numerator 0.035 over denominator 0.035 end fraction end style = 1 begin mathsize 14px style fraction numerator 0.245 over denominator 0.035 end fraction end style = 7

  • So, the value of x is 7
  • Therefore, the formula of the hydrated salt is CuSO4•7H2O

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.