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2023 O Level Chem P1 Q5 — Acids, Bases and Redox Solution

Acids, Bases and Redox 2023 O Level Pure Chemistry · Paper 1 · Q5

Question

Sulfur dioxide, SO₂, is dissolved in a little water to make a solution. The pH of the solution is tested. Sulfur dioxide is then bubbled into acidified potassium manganate(VII). Which row identifies the pH of the solution and the observation with acidified potassium manganate(VII)? Question diagram

Question diagram

A
B
C
D

Correct answer: A

Video Solution

Explanation

Answer: A. \(\text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3\) — sulfurous acid is acidic, so the solution has pH below 7; pH 3 fits (pH 9 would mean an alkaline solution, which is wrong). This rules out options C and D.

\(\text{SO}_2\) (or \(\text{H}_2\text{SO}_3\)) is a reducing agent. Acidified potassium manganate(VII) is a purple oxidising agent. When the reducing agent reacts with it, \(\text{MnO}_4^-\) (purple) is reduced to \(\text{Mn}^{2+}\) (colourless): the solution is therefore decolourised. 'No change' (options B and D) cannot happen with a reducing gas being bubbled through.

So pH 3 plus decolourisation — only row A combines both correctly.

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