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Oxidising and Reducing Nature

A black compo...

Question

A black compound $(A)$ in solid state is fused with $K O H$ and $K C l O_{3}$ . The aqueous extract of fused mass is green color solution $(B)$ . On passing $C O_{2}$ gas through it, pink color of $(C)$ is noticed along with some black insoluble mass of $(A)$ . The pink colored solution is decolorized by $F e^{2 +}$ in acidic medium. What is the oxidation state of central metal in $(C)$ ?

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Solution

The corresponding chemical reactions are
$3 M n O_{2} A + 6 K O H + K C l O_{3} Δ - \to 3 K_{2} M n O_{4} B + K C l + 3 H_{2} O$

In presence of $C O_{2},$ the medium becomes acidic
$3 M n O_{4}^{2 -} + 4 H^{+} - \to M n O_{2} + 2 M n O_{4}^{-} C + 2 H_{2} O$ .
$M n O_{4}^{-} + 5 F e^{2 +} + 8 H^{+} - \to M n^{2 +} + 5 F e^{3 +} + 4 H_{2} O$ .
So we need to find the oxidation state of $M n$ in $M n O_{4}^{-}$
Hence, oxidation state is
$x + 4 (- 2) = - 1$
$\Rightarrow x = + 7$

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