`KMnO_4` is prepared from the mineral pyrolusite, `MnO_2` (deep purple colour). It acts as an oxidising agent in the neutral, alkaline as well as acidic medium in acidic medium it is used in volumetric analysis for estimation of `Fe^(2+)`, `Cr_2O_4^(2-)` salts etc. The titrations are carried out in presence of `H_2SO_4`. However, before using it as a titrant, it is first standardised with standard oxalic acid solution or Mohr's salt solution . In one of the experiments on titration 26.8g of dry pure sodium oxalate `(Mw=123gmol^-1)` was dissolved in 1L of distilled water and then 100 mL of `2MH_2SO_4` were added. The solution was cooled. Now to this solution `0.1MKMnO_4` solution was added till a very faint pink colour persisted.
Q. Mohr's salt, `FeSO_4(NH_4)_2SO_46H_2O`, is preffered over `FeSO_4.7H_2O` for standardisation of `KMnO_4` solution because.

Options

A. Mohr's salt is a double salt while ferrous sulphate is a single salt.
B. Mohr's salt is not hygroscopic but `FeSO_4.7H_2O` 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 `H_2SO_4`.

Answer

c

Explanation

`FeSO_4.7H_2O` contains some ferric ions due to arial oxidation.