A research team develops a salt substitute containing potassium chloride. Fig. 1 shows a section through its ionic lattice. The team dissolves measured mass...
Question 1 Report
A research team develops a salt substitute containing potassium chloride. Fig. 1 shows a section through its ionic lattice. The team dissolves measured masses in water at 25 °C and records the mass that dissolves in 100 g of water. Table 1 compares potassium chloride with a covalent substance, sucrose. The results are useful when preparing a concentrated solution for food testing.
| substance | mass dissolved in 100 g water / g |
|---|---|
| potassium chloride | 34 |
| sucrose | 211 |
(a) Name the positive ion in Fig. 1. [1]
(b) Use Table 1 to state which substance is more soluble in water. [1]
(c) Explain why potassium chloride has an overall charge of zero. [2]
(d) Complete the formula for potassium chloride: K__. [1]
A geologist identifies a white mineral found near an old sea bed as sodium sulfate. Fig. 1 shows the ions in its formula, including the polyatomic sulfate ion. Sodium sulfate can be obtained by reacting sulfuric acid with sodium hydroxide solution. The geologist needs the correct formula before calculating the mass needed for a water-quality test.
(a) Name the negative ion shown in Fig. 1. [1]
(b) Complete the formula of sodium sulfate: Na__SO4. [1]
(c) Explain why two sodium ions are needed for each sulfate ion. [2]
(d) What type of bonding holds sodium and sulfate ions together? [1]
Answer Details
Potassium chloride
- (a) The positive ion is the potassium ion, \(\mathrm{K^+}\). [1 mark]
- (b) Sucrose is more soluble: \(211\text{ g}\) dissolves in \(100\text{ g}\) of water compared with \(34\text{ g}\) for potassium chloride. [1 mark]
- (c) Potassium chloride contains positive potassium ions and negative chloride ions. Their equal and opposite charges balance, so the compound has an overall charge of zero. [2 marks]
- (d) The formula is \(\mathrm{KCl}\). [1 mark]
Sodium sulfate
- (a) The negative ion is the sulfate ion. [1 mark]
- (b) The formula is \(\mathrm{Na_2SO_4}\). [1 mark]
- (c) Each sodium ion has charge \(+1\). Two sodium ions provide \(+2\), which balances the \(2-\) charge of one sulfate ion. [2 marks]
- (d) Sodium and sulfate ions are held together by ionic bonding. [1 mark]
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