2.2 Ionic compounds and their formulae
6 exam questions worth 10 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What is the formula of magnesium chloride?
Choose one answer.
Correct Not quite: the answer is MgCl2
Mark scheme
B
Model answer
B: one Mg2+ ion balances two Cl− ions, so MgCl2.
Aluminium ions are \mathrm{Al^{3+}} and oxide ions are \mathrm{O^{2-}}.
Work out the formula of aluminium oxide. Show your working.
Mark scheme
- 2 \mathrm{Al^{3+}} = 6+ balances 3 \mathrm{O^{2-}} = 6− (1)
- Al2O3 (1)
Model answer
Two aluminium ions give 6+ and three oxide ions give 6−, so the formula is Al2O3.
Write the formula of ammonium sulfate.
Mark scheme
- (NH4)2SO4 (1)
Model answer
(NH4)2SO4
Explain the difference between sodium sulfide and sodium sulfate.
Mark scheme
- sodium sulfide contains only sodium and sulfur (1)
- sodium sulfate also contains oxygen / contains the sulfate ion \mathrm{SO_4^{2-}} (1)
Model answer
Sodium sulfide contains only sodium and sulfur (Na2S). Sodium sulfate also contains oxygen, in sulfate ions (Na2SO4).
Describe the structure of solid sodium chloride.
Mark scheme
- giant (ionic) lattice (1)
- regular arrangement of alternating \mathrm{Na^+} and \mathrm{Cl^-} ions (1)
- held by strong electrostatic forces of attraction in all directions (1)
Model answer
Sodium chloride is a giant ionic lattice: a regular arrangement of alternating \mathrm{Na^+} and \mathrm{Cl^-} ions held together by strong electrostatic forces acting in all directions.
Give one limitation of using a ball and stick model to represent sodium chloride.
Mark scheme
any one from:
- there are no sticks/bonds like this between ions (1)
- the gaps between the ions are exaggerated (1)
- only a small part of the lattice is shown (1)
Model answer
The ions are not really joined by sticks, and the gaps between them are much too large.
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