Question

When potassium permanganate is titrated against ferrous ammonium sulphate, the equivalent weight of potassium permanganate is:
a.) molecular weight / 10
b.) molecular weight / 5
c.) molecular weight / 2
d.) molecular weight

Answer 1

Hint : Equivalent is the quantity that combines with or replaces 1 g of hydrogen or 8 g of oxygen. And titration is a technique where a solution of known concentration is used to determine the concentration of the unknown solution.

Complete step by step solution :
Equivalent weight can be defined as the one equivalent which is the mass of a given substance which combines with or replaces a fixed quantity of another substance. It is the mass of a substance that combines with or is chemically equivalent to one gram of hydrogen or eight grams of oxygen.
The titration given in the question can be written as follows:

$2KMn{ O }_{ 4 }\quad +\quad 10FeS{ O }_{ 4 }.{ (N{ H }_{ 4 }) }_{ 2 }.6{ H }_{ 2 }O\quad +\quad 8{ H }_{ 2 }O\quad \longrightarrow \quad { K }_{ 2 }S{ O }_{ 4 }\quad +\quad 2MnS{ O }_{ 4 }\quad +\quad 5{ Fe }_{ 2 }{ (S{ O }_{ 4 }) }_{ 3 }\quad +\quad 10{ (N{ H }_{ 4 }) }_{ 2 }S{ O }_{ 4 }\quad +\quad 68{ H }_{ 2 }O$

In this reaction, Mn oxidizes Fe and gets itself reduced. The oxidation state of Mn before the reaction is +7, and after the reaction, it becomes +2.

$K\overset { +7 }{ Mn } { O }_{ 4 }\quad \longrightarrow \quad \overset { +2 }{ Mn }$

The n-factor is the change in the oxidation state of any compound.

n-factor of Mn = Change in oxidation state of Mn = 5
The equivalent weight is molecular weight divided by the n-factor.
$Equivalent\quad weight\quad =\quad \cfrac { Molecular\quad Weight }{ n-factor }$
Substituting the values, we get,
$Equivalent\quad weight\quad =\quad \cfrac { Molecular\quad Weight }{ 5 }$
Therefore, the equivalent weight of potassium permanganate is $\cfrac {Molecular Weight}{5}$.
So, the correct answer is “Option B”.

Note :
Students tend to understand the molecular weight and the equivalent weight of a compound as equal. But they are different from each other. Molecular weight is the sum of the weights of the individual atoms in the molecule whereas, the equivalent weight is molecular weight divided by the n-factor.