Question

Which of the following statements are true for Mohr’s salt?
A.It decolorizes ${\rm{KMn}}{{\rm{O}}_{\rm{4}}}$ solution
B.It is a double salt
C.It is a colorless salt
D.It is a primary standard substance

Answer 1

We know that, Mohr’s salt is an inorganic compound whose formula is ${\left( {{\rm{N}}{{\rm{H}}_{\rm{4}}}} \right)_2}{\rm{Fe}}{\left( {{\rm{S}}{{\rm{O}}_{\rm{4}}}} \right)_2}{\left( {{{\rm{H}}_{\rm{2}}}{\rm{O}}} \right)_6}$. It is one of the reagents which is very much used in laboratories because of its property to easily undergo crystallization.

Complete step by step answer:
Here, we discuss all the options one by one.

Option A says that Mohr’s salt decolorizes potassium permanganate solution. Mohr’s salt is used as primary standard in redox titration of potassium permanganate. In this reaction, the potassium permanganate undergoes reduction to form ${\rm{M}}{{\rm{n}}^{2 + }}$ ion. Therefore, the colour of potassium permanganate solution vanished. The reaction is,
${\rm{Mn}}{{\rm{O}}_{\rm{4}}}^ - + 8{{\rm{H}}^ + } + 5{\rm{F}}{{\rm{e}}^{2 + }} \to {\rm{M}}{{\rm{n}}^{2 + }} + 5{\rm{F}}{{\rm{e}}^{3 + }} + 4{{\rm{H}}_{\rm{2}}}{\rm{O}}$
So, option A is correct regarding Mohr’s salt.
Option B says Mohr’s salt is a double salt. A double salt is that salt which contains more than one cation or anion. Mohr’s salt $\left( {{{\left( {{\rm{N}}{{\rm{H}}_{\rm{4}}}} \right)}_2}{\rm{Fe}}{{\left( {{\rm{S}}{{\rm{O}}_{\rm{4}}}} \right)}_2}{{\left( {{{\rm{H}}_{\rm{2}}}{\rm{O}}} \right)}_6}} \right)$ consists of two cations, namely ammonium ion $\left( {{\rm{N}}{{\rm{H}}_{\rm{4}}}^ + } \right)$ and ferrous ion $\left( {{\rm{F}}{{\rm{e}}^{{\rm{2 + }}}}} \right)$. So, it is double salt. Therefore, option B is also correct.
Option C says Mohr’s salt is a colourless salt. This statement is not correct as it is light green in color.
Option D says it is used as the primary standard substance. This statement is correct as it is used as the primary standard in volumetric analysis.

So, options A, B and D are correct.

Note:
The Mohr’s salt can be manufactured by dissolving equimolar mixture of ammonium sulphate and hydrated ferrous sulphate in water in presence of little sulphuric acid. The sulphuric acid is required in the reaction because it prevents the hydrolysis of salt. Subject to crystallization, ferrous ammonium sulphate (Mohr’s salt) separates from the solution.