A chemist can choose from the following ions.
| Positive ions available | Negative ions available |
|---|---|
| sodium, Na+\mathrm{Na^{+}}Na+ | chloride, Cl−\mathrm{Cl^{-}}Cl− |
| magnesium, Mg2+\mathrm{Mg^{2+}}Mg2+ | oxide, O2−\mathrm{O^{2-}}O2− |
| aluminium, Al3+\mathrm{Al^{3+}}Al3+ |
The required ionic compound has the general formula M2X\mathrm{M_2X}M2X, where M\mathrm{M}M represents the positive ion and X\mathrm{X}X represents the negative ion.
What pair of ions should the chemist select?
Na+\mathrm{Na^{+}}Na+ and Cl−\mathrm{Cl^{-}}Cl−
Na+\mathrm{Na^{+}}Na+ and O2−\mathrm{O^{2-}}O2−
Al3+\mathrm{Al^{3+}}Al3+ and O2−\mathrm{O^{2-}}O2−
Mg2+\mathrm{Mg^{2+}}Mg2+ and Cl−\mathrm{Cl^{-}}Cl−
134 exam-style questions on OCR GCSE Chemistry 3.1 Introducing chemical reactions, covering 3.1.1 Writing formulae and balanced chemical equations, 3.1.2 Formulae of compounds from ions, and state symbols, 3.1.3 Conservation of mass and observed changes in mass, 3.1.4 Half equations and balanced ionic equations, 3.1.5 The mole and the Avogadro constant, and 3.1.6 Stoichiometry, limiting reactants and reacting masses. Each one has a worked solution and a mark scheme showing where the marks go.