A 0.001 mol L⁻¹ solution of hydrochloric acid and a 0.056 mol L⁻¹ solution of ethanoic acid both have a pH of 3.0 - HSC - SSCE Chemistry - Question 21 - 2010 - Paper 1

Ethanoic acid (CH₃COOH) is identified as a weak acid. Unlike strong acids, weak acids do not fully ionize in solution. Therefore, at a concentration of 0.056 mol L⁻¹, only a fraction of the CH₃COOH molecules will dissociate to produce H⁺ ions.

The pH of both solutions is identical at 3.0, indicating that the concentration of H⁺ ions, derived from both solutions, is the same. For HCl, the [H⁺] concentration is 0.001 mol L⁻¹. For the ethanoic acid solution to achieve the same pH, the effective concentration of H⁺ ions must also equal $[H^+] = 10^{-3}$ mol L⁻¹. This can happen when a higher concentration of ethanoic acid dissociates partially to yield sufficient H⁺ ions, which is consistent with the weaker ionization nature of CH₃COOH.