Which of the following statements is/are true for Mohr’s salt?

It decolorizes $K M n O_{4}$ solution

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It is a double salt

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It is a colourless salt

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It is a primary standard substance

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Solution

The correct option is D
It is a primary standard substance

Properties of Mohr's salt:
(a) In the titration of Mohr's salt with $K M n O_{4}$ solution, it acts as the reducing agent and potassium permanganate acts as the oxidising agent,
$2 K M n O_{4} + 10 F e S O_{4} (N H_{4})_{2} S O_{4} .6 H 2 O + 8 H_{2} S O_{4} \to K_{2} S O_{4} + 2 M n S O_{4} + 5 F e_{2} (S O_{4})_{3} + 10 (N H_{4})_{2} S O_{4} + 68 H_{2} O$

Since $M n O_{4}^{-}$ oxidises to $M n^{2 +}$ it decolorises.
(b)Mohr's salt $F e S O_{4} (N H_{4})_{2} S O_{4} .6 H 2 O$ contains two different cations $F e^{2 +}$ and $N H_{4}^{+}$ , and hence, it is classified as a double salt.
(c)It is a blue-green solid.
(d)It is used as a primary standard in volumetric analysis.

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