COMEDK 2025 Morning Solved Paper

Concept:Dilute HCl cannot be used because chloride ions ($\mathrm{Cl}^-$) are also oxidised by $\mathrm{KMnO}_4$, interfering with the titration of oxalic acid.Explanation:In the redox titration, $\mathrm{KMnO}_4$ oxidises oxalic acid ($\mathrm{H}_2\mathrm{C}_2\mathrm{O}_4$) to $\mathrm{CO}_2$ in acidic medium (provided by dil. $\mathrm{H}_2\mathrm{SO}_4$). If dil. HCl is used instead, $\mathrm{Cl}^-$ ions from HCl undergo oxidation by $\mathrm{MnO}_4^-$ to form $\mathrm{Cl}_2$ gas. The reaction is: $$2\mathrm{MnO}_4^- + 10\mathrm{Cl}^- + 16\mathrm{H}^+ \rightarrow 2\mathrm{Mn}^{2+} + 5\mathrm{Cl}_2 + 8\mathrm{H}_2\mathrm{O}$$ Thus, $\mathrm{KMnO}_4$ is consumed by $\mathrm{Cl}^-$ as well, leading to an incorrect titre value. The $\mathrm{Cl}^-$ ion acts as a reducing agent itself, similar to oxalic acid. Hence, the $\mathrm{Cl}^-$ ions compete with oxalic acid for the oxidant, making the titration unreliable.Answer:Option A: HCl can also reduce $\mathrm{MnO}_4^-$ to $\mathrm{Mn}^{2+}$.