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Question

Which of the following sets includes all the species that will change the orange colour of $K_{2}{Cr}_{2}O_{7}$ in acidic medium?

Options

A.${Fe}^{2 +},{Sn}^{2 +},I^{-},S^{2 -}$
B.$S^{2 -},{Fe}^{3 +},I^{-},C_{2}O_{4}^{2 -}$
C.${Fe}^{2 +},{NO}_{2}^{-},{SO}_{2},{Sn}^{4 +}$
D.${Fe}^{3 +},{SO}_{4}^{2 -},S^{2 -},{Sn}^{4 +}$

Solution

Sol. [Key Concept: A species decolourises dichromate only if it can be oxidised — pick the reducing species.]

Reducing agent $+ {KMnO}_{4}\overset{\phantom{\begin{matrix} \text{~}\text{acidic}\text{~} \\ \text{~}\text{medium}\text{~} \end{matrix}\ }}{\rightarrow}{Mn}^{+ 2} +$ Product

Given Reducing Agent

${Fe}^{+ 2} \rightarrow {Fe}^{+ 3},{Sn}^{+ 2} \rightarrow {Sn}^{+ 4}$,$I^{-} \rightarrow I_{2},S^{- 2} \rightarrow \text{ }S$

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