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Formulae, Equations and Amounts of Substance

Hydrated salts and water of crystallisation

  • 3 things to know
  • Unit AS 1

Overview

Term water of crystallisation, anhydrous compound and colour change seen when hydrated copper(II) sulfate is heated strongly.

What you need to know

Answer each one before you reveal it.

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Numbers

Hydrated magnesium sulfate is found to be 51.1% water by mass. Calculate the number of moles of water of crystallisation.

Key term

Define the term water of crystallisation.

Key term

What is meant by an anhydrous compound?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Define the term water of crystallisation.

    Model answer

    Water of crystallisation is water that is chemically bound into the crystal structure of a salt in a fixed molar proportion.

  2. 2

    What is meant by an anhydrous compound?

    Model answer

    An anhydrous compound is one that contains no water of crystallisation.

  3. 3

    What does it mean to describe a salt as hydrated?

    Model answer

    A hydrated salt is one whose crystals contain water of crystallisation as part of their structure.

  4. 4

    How is water of crystallisation shown in the formula of a salt?

    Model answer

    It is written after the formula of the salt separated by a dot, as in CuSO₄·5H₂O, where the number gives the moles of water per mole of salt.

  5. 5

    Describe the colour change seen when hydrated copper(II) sulfate is heated strongly.

    Model answer

    The blue hydrated crystals turn white as the anhydrous copper(II) sulfate is formed and the water is driven off.

  6. 6

    Why is a hydrated salt heated to constant mass rather than for a fixed time?

    Model answer

    Heating until two successive weighings agree shows that all the water of crystallisation has been driven off and no more mass can be lost.

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