d and f-Block ElementsMedium

Question

A paper dipped in a dil. $H_{2}{SO}_{4}$ solution of ' X ' upon treatment with ${SO}_{2}$ gas turns into green. The compound ' X ' is :

Options

A.KI-starch
B.${KMnO}_{4}$
C.$Pb\left( {CH}_{3}COO \right)_{2}$
D.$K_{2}{Cr}_{2}O_{7}$

Solution

Sol. [Key Concept: ${SO}_{2}$ acts as a reducing agent, reducing orange $K_{2}{Cr}_{2}O_{7}$ to green ${Cr}_{2}({SO}_{4})_{3}$ (Cr+3).]

${SO}_{2}$ gas acts as reducing agent.

$$K_{2}{Cr}_{2}O_{7} + {SO}_{2}\overset{\phantom{H_{2}{SO}_{4}\ }}{\rightarrow}\underset{\text{~}\text{Green solution}\text{~}}{{Cr}_{2}}\left( {SO}_{4} \right)_{3} + {SO}_{4}^{2 -}$$

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