`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. The volume of `KMnO_4` solution that must have been added to obtain the faint pink colour at the end point must be

Options

A. 100 mL
B. 200 mL
C. 400 mL
D. 800 mL

Answer

d

Explanation

First Method:
Moles of `(COONa)_2=(26.8)/(134)=0.2`
The balanced equation for the reaction involved is
`2MnO_4^(ɵ)+16H^(o+)+5C_2O_4^(2-)to2Mn^(2+)8H_2O+10CO_2`
Moles of `KMnO_4` used `=(2)/(5)xx0.2=0.08` mol
As `KMnO_4` solution used is 0.1 M, i.e., 0.1 mol are present in 1000 mL, therefore 0.08 mol will be present in 800 mL.
Second Method:
`mEqofMnO_4^(ɵ)=mEq.C_2O_4^(2-)`
`(0.1Mxx5)NxxV=((26.8)/(134)xx2)xx10^3mEq`.
`0.5VmEq.=400mEq` `V=800mL`