2017 H2 Chem P1 Q28 — Electrochemical cell Solution

The answer is B (aqueous hydrogen chloride, 1 mol \(\text{dm}^{-3}\)).
A liquid conducts electricity when it contains mobile ions, and the more ions there are, the better it conducts.
HCl is a strong acid. In water it is fully ionised: \(\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-\), so 1 mol \(\text{dm}^{-3}\) HCl(aq) contains 1 mol \(\text{dm}^{-3}\) of \(\text{H}_3\text{O}^+\) and 1 mol \(\text{dm}^{-3}\) of \(\text{Cl}^-\), a total of 2 mol \(\text{dm}^{-3}\) of mobile ions, the highest of the four liquids.
• A is wrong: ethanoic acid is a weak acid and only partially ionises. With \(K_\text{a} = 1.8 \times 10^{-5}\) (\(\text{p}K_\text{a}\) 4.76), \([\text{H}^+] = \sqrt{1.8 \times 10^{-5} \times 1} = 4.2 \times 10^{-3} \text{ mol dm}^{-3}\), so there are far fewer ions than in 1 mol \(\text{dm}^{-3}\) HCl.
• C is wrong: pure ethanoic acid consists of covalent molecules. With no water present to hydrate ions, it undergoes almost no self-ionisation, so it has very few mobile ions.
• D is wrong: pure liquid hydrogen chloride consists of covalent HCl molecules. Without water it does not ionise, so it has virtually no mobile ions.