Question

What is the equivalent mass of an oxidising agent, KMnO₄, in a strongly alkaline medium?

1. 52.67
2. 39.5
3. 158
4. 31.6

Answer 1

Correct Answer - Option 3 : 158

Concept:

• Oxidizing agents change the oxidation number from low to high in a substance it is oxidizing.
• It takes up electrons from the compound and then reduces itself.
• KMnO₄ as an oxidizing agent in basic medium as-

MnO4- + 2 H2O + 3 e- → MnO2(s) + 4OH- (gained 3 electrons)

• KMnO4 acts as an oxidizing agent in acidic medium as-

MnO4- + 8 H+ + 5 e- → Mn2+ + 4 H2O (gained 5 electrons from reductant)

• In a strongly alkaline medium, the ion changes to 

MnO4- + 1 e^- → MnO₄^2-

Equivalent mass:

• It is the amount of substance that combines with 1g of hydrogen or 8 g of oxygen or 35.5g of chlorine.
• The formula for calculating the equivalent mass of a substance undergoing redox reaction is given by:

\(Equivalent\;weight = \frac{{formula\;mass}}{{no\;of\;electrons\;exchanged}}\)​

Explanation:

• In a strongly alkaline medium, the ion changes to 

MnO4- + 1 e- → MnO42-

• The oxidation state changes from +7 to +6.
• Hence, the number of electrons exchanged = 1.
• The molar mass of potassium permanganate is 158g.

\(Equivalent\;weight\;of\;KMnO_4 = \;\frac{{158}}{1} = 158\)

Hence, the equivalent mass of an oxidizing agent, KMnO4, in a strongly alkaline medium is 158.