Question

Mohr’s salt ${\text{FeS}}{{\text{O}}_4} \cdot {\left( {{\text{N}}{{\text{H}}_4}} \right)_2} \cdot {\text{S}}{{\text{O}}_4} \cdot {\text{6}}{{\text{H}}_2}{\text{O}}$ is preferred over ${\text{FeS}}{{\text{O}}_{\text{4}}} \cdot {\text{7}}{{\text{H}}_{\text{2}}}{\text{O}}$ for standardization of ${\text{KMn}}{{\text{O}}_{\text{4}}}$ solution because:
A. Mohr’s salt is a double salt while ferrous sulphate
B. Mohr’s salt is not hygroscopic but ${\text{FeS}}{{\text{O}}_{\text{4}}} \cdot {\text{7}}{{\text{H}}_{\text{2}}}{\text{O}}$ is hygroscopic.
C. Mohr’s salt contains only ferrous ions whereas ferrous sulphate contains some ferric ions.
D. Mohr’s salt solution can be titrated even in the absence of ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$.

Answer 1

Hint: In analytical chemistry, we use a standardized solution. A standardized solution is the one containing a known concentration of an element or substance. In a standard solution, a known weight of solute is dissolved to make a specific volume.

Complete step by step solution:
For standardization of potassium permanganate $\left( {{\text{KMn}}{{\text{O}}_{\text{4}}}} \right)$ solution we use Mohr’s salt having formula ${\text{FeS}}{{\text{O}}_4} \cdot {\left( {{\text{N}}{{\text{H}}_4}} \right)_2} \cdot {\text{S}}{{\text{O}}_4} \cdot {\text{6}}{{\text{H}}_2}{\text{O}}$. Mohr’s salt is also known as ammonium ferrous sulphate or ammonium iron (II) sulphate. It contains ferrous $\left( {{\text{F}}{{\text{e}}^{2 + }}} \right)$ ion and ammonium $\left( {{\text{NH}}_4^ + } \right)$ ion.
In the titration, during standardization of ${\text{KMn}}{{\text{O}}_{\text{4}}}$, ${\text{KMn}}{{\text{O}}_{\text{4}}}$ itself acts as an oxidising agent.
We know that the oxidising agent oxidises the other species and itself gets reduced. Thus, the manganese ion in ${\text{KMn}}{{\text{O}}_{\text{4}}}$ which is in $ + {\text{7}}$ oxidation state gets reduced to manganese ion in $ + {\text{2}}$ oxidation state. The reduction half reaction is as follows:
$2{\text{KMn}}{{\text{o}}_4} + {\text{3}}{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}} \to {{\text{K}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}} + {\text{2MnS}}{{\text{O}}_{\text{4}}} + {\text{3}}{{\text{H}}_{\text{2}}}{\text{O}} + {\text{5}}\left( {\text{O}} \right)$
The Mohr’s salt gets oxidised. The ferrous ion $\left( {{\text{F}}{{\text{e}}^{2 + }}} \right)$ in Mohr’s salt gets oxidised to ferric ion $\left( {{\text{F}}{{\text{e}}^{3 + }}} \right)$. The oxidation half reaction is as follows:
${\text{FeS}}{{\text{O}}_4} \cdot {\left( {{\text{N}}{{\text{H}}_4}} \right)_2} \cdot {\text{S}}{{\text{O}}_4} \cdot {\text{6}}{{\text{H}}_2}{\text{O}} + {{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}} + {\text{5}}\left( {\text{O}} \right) \to {\text{F}}{{\text{e}}_{\text{2}}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_3} + {\text{2}}{\left( {{\text{N}}{{\text{H}}_4}} \right)_2}{\text{S}}{{\text{O}}_4} + 1{\text{3}}{{\text{H}}_{\text{2}}}{\text{O}}$
The ferrous ion from Mohr’s salt gets oxidised and the manganese ion gets reduced.
The pink coloured of potassium permanganate becomes colourless when the reduction of manganese ion occurs. Thus, potassium permanganate acts as a self-indicator.
The titration involves the conversion of $F{e^{ + 2}}$to $F{e^{ + 3}}$and if ferrous sulphate $\left( {{\text{FeS}}{{\text{O}}_{\text{4}}} \cdot {\text{7}}{{\text{H}}_{\text{2}}}{\text{O}}} \right)$ is used then the conversion will give an error in the determination.
Thus, Mohr’s salt ${\text{FeS}}{{\text{O}}_4} \cdot {\left( {{\text{N}}{{\text{H}}_4}} \right)_2} \cdot {\text{S}}{{\text{O}}_4} \cdot {\text{6}}{{\text{H}}_2}{\text{O}}$ is preferred over ${\text{FeS}}{{\text{O}}_{\text{4}}} \cdot {\text{7}}{{\text{H}}_{\text{2}}}{\text{O}}$ for standardization of ${\text{KMn}}{{\text{O}}_{\text{4}}}$ solution because Mohr’s salt contains only ferrous ions whereas ferrous sulphate contains some ferric ions.

Thus, the correct option is (C) Mohr’s salt contains only ferrous ions whereas ferrous sulphate contains some ferric ions.

Note:
Mohr’s salt should be carefully handled because inhalation of Mohr’s salt causes irritation in the nose and throat. Ingestion of Mohr’s salt causes nuisance in mouth and stomach.