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

HCN(aq) is a weak acid, which means it only partially ionizes in solution. The ionization can be represented by the following chemical equation:

$HCN(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + CN^-(aq)$

In contrast, HCl(aq) is a strong acid and fully ionizes:

$HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)$

Since HCl ionizes completely, the concentration of hydrogen ions [H⁺] in the HCl solution is much higher compared to that in the HCN solution. The pH is defined as:

$pH = -\log[H^+]$

As a result, HCN, which has a lower concentration of hydronium ions compared to HCl, will yield a higher pH value. Therefore, the HCN(aq) solution is less acidic and has a higher pH than the HCl(aq) solution.