A colorless solid salt (A) reacts with dilute to produce a colorless gas (B). Gas (B) reduces acidif — Qualitative and Quantitative Analysis Chemistry Question
Question
A colorless solid salt (A) reacts with dilute $H_2SO_4$ to produce a colorless gas (B). Gas (B) reduces acidified $K_2Cr_2O_7$ solution to a green solution. Furthermore, when gas (B) is passed through an aqueous solution of $H_2S$, a milky white turbidity is observed. Identify gas (B).
Answer: B
💡 Solution & Explanation
The salt contains a sulphite ($SO_3^{2-}$) ion. Upon reaction with dilute acid, it releases $SO_2$ gas. $SO_2$ acts as a reducing agent, turning orange acidified $K_2Cr_2O_7$ into green $Cr^{3+}$ ions. Additionally, $SO_2$ reacts with $H_2S$ (where $SO_2$ acts as an oxidizing agent) to precipitate elemental sulphur, causing the milky white turbidity: $2H_2S + SO_2 \rightarrow 3S \downarrow + 2H_2O$.
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