Chemistry, Third edition

CALCULATING AMOUNTS FROM EQUATIONS

Water of crystallization

The formulae of some ionic compounds may be written as AB ·nH 2 O, where

‘·nH 2 O’ indicates that nmolecules of water are associated with each AB unit within

the crystal lattice. Because the water is within the lattice, the substance is perfectly

dry to the touch but it contains water of crystallizationand is said to be hydrated.

The water may be driven off by strong heating (dehydration) and the residue, that

now does not contain water of crystallization, is termed anhydrous.

Blue copper(II) sulfate crystals (CuSO 4 ·5H 2 O) become white, anhydrous

copper(II) sulfate on heating:

CuSO 4 · 5H 2 O(s)CuSO 4 (s)5H 2 O(l)

blue white

If water is added to anhydrous copper(II) sulfate, it turns blue again.

Other common substances that contain water of crystallization are

sodium carbonate decahydrate (washing soda): Na 2 CO 3 · 10H 2 O

magnesium sulfate heptahydrate (Epsom salts): MgSO 4 · 7H 2 O

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Substances that contain water of

crystallization

The dehydration of copper(II) sulfate by heating is a

‘stepwise’ process

(i) First, the pentahydrate loses water to become first the

trihydrate.

(ii)Then the monohydrate is formed.

(iii)Finally the anhydrous salt remains.

(iv)If the anhydrous salt is heated very strongly, it

decomposes to copper(II) oxide and sulfur trioxide.

Write balanced equations for all of these reactions.

Exercise 8H

Calculating amounts from equations

Calculating masses

You have already seen that chemical equations give the following information:

1.The chemical composition of the reactants and products in a reaction.

2.The simplest whole-number relationship of the molecules involved in the

reaction.

Using the mole concept, together with the atomic masses of the elements involved,

we are now in a position to work out the masses of substances involved in chemical

reactions.

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