Edexcel IGCSE Chemistry 1.39 Ionic Compound Formulae

Practise balancing ion charges to write ionic formulae and naming compounds containing simple or polyatomic ions.

Exam points

choose ion subscripts so total positive and negative charge equals zero
place a polyatomic ion in brackets when more than one whole ion is required
name the cation before the anion and preserve variable oxidation states where supplied

[Maximum number: 2]

Sodium hydroxide dissolves in water, forming a strongly alkaline solution.
Ammonia dissolves in water, forming a slightly less alkaline solution.

Ammonium sulfate is used by gardeners as a fertiliser because it contains nitrogen.

Explain why the chemical formula of ammonium sulfate is $\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}$ Refer to the charges on the ions in your answer.

M1 ammonium ion is charged $1^{+} / \mathrm{NH}_{4}{ }^{+}$

AND sulfate ion is charged $2^{-} / \mathrm{SO}_{4}{ }^{2-}$

M2 so charges balance/cancel (each other) OWTTE

ALLOW so that ammonium sulfate has no overall charge

M2 not dep on M1

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1.39 Ionic compound formulae question 1