The pH of two aqueous solutions was compared - HSC - SSCE Chemistry - Question 25 - 2022 - Paper 1

HCN(aq) is a weak acid, so it partially ionizes in solution:

$\text{HCN}(aq) + \text{H}_2\text{O}(l) \rightleftharpoons \text{H}_3\text{O}^+(aq) + \text{CN}^-(aq)$

On the other hand, HCl(aq) is a strong acid and ionizes completely. Therefore, the concentration of hydronium ions, [H⁺], in the HCl(aq) solution is higher than that in the HCN(aq) solution. The pH is calculated as follows:

$\text{pH} = -\log[\text{H}^+]$

Since the concentration of hydronium ions in HCN is lower, the pH of the HCN(aq) solution will be higher than that of the HCl(aq) solution.