The salt that reacts with dilute hydrochloric acid to produce a pungent smelling gas which decolourizes…

Analysis

To solve this question, we need to identify which of the given salts reacts with dilute hydrochloric acid (HCl) to produce a gas that:

1. Has a pungent smell.
2. Decolourizes acidified purple potassium tetraoxomanganate (VII) ($KMnO_4$).

Concept Explanation

• Na$_2$SO$_4$ (Sodium tetraoxosulphate VI): Does not react with dilute HCl to produce a gas.
• Na$_2$SO$_3$ (Sodium trioxosulphate IV): Reacts with dilute HCl to produce sulphur (IV) oxide ($SO_2$) gas. $SO_2$ is a colourless gas with a characteristic pungent, choking smell (like burnt matches). It acts as a reducing agent, which causes the purple $KMnO_4$ to turn colourless.
• Na$_2$S (Sodium sulphide): Reacts with dilute HCl to produce hydrogen sulphide ($H_2S$) gas. While $H_2S$ has a smell, it smells like rotten eggs, not a pungent choking smell, and it produces a yellow precipitate of sulphur when reacting with $KMnO_4$.
• Na$_2$CO$_3$ (Sodium trioxocarbonate IV): Reacts with dilute HCl to produce carbon (IV) oxide ($CO_2$). $CO_2$ is odourless and does not decolourize acidified $KMnO_4$.

Step-by-Step Solution

1. Reaction: When sodium trioxosulphate (IV) reacts with hydrochloric acid: $$Na_2SO_3(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2O(l) + SO_2(g)$$
2. Gas Identification: The gas produced is $SO_2$.
3. Chemical Test: $SO_2$ is a strong reducing agent. When bubbled through acidified $KMnO_4$, it reduces the manganese from the +7 oxidation state (purple) to the +2 oxidation state (colourless): $$5SO_2 + 2MnO_4^- + 2H_2O \rightarrow 5SO_4^{2-} + 2Mn^{2+} + 4H^+$$ This confirms the decolourisation process.

Conclusion

The salt that satisfies both conditions (pungent smell and decolourisation of $KMnO_4$) is sodium trioxosulphate (IV).

Exam Tip

Whenever you see "decolourizes acidified $KMnO_4$," immediately think of reducing agents. Common reducing agents in your syllabus include $SO_2$, $H_2S$, and $Fe^{2+}$ salts. If the question mentions a "pungent smell," $SO_2$ is almost always the intended answer!