The formula mass of Mohr's salt is 392. The iron present in it is oxidised by KMnO4 in acid medium. The equivalent mass of Mohr's salt is (A) 392 (B) 31.6 (C) 278 (D) 156

To determine the equivalent mass of Mohr's salt, we first need to understand its composition and how it reacts in an acidic medium, especially with potassium permanganate (KMnO4). Mohr's salt, which is also known as ferrous ammonium sulfate, has the formula FeSO4·(NH4)2SO4·6H2O. The molar mass of Mohr's salt is indeed 392 g/mol, as you mentioned.

Understanding Equivalent Mass

The equivalent mass of a substance is calculated by dividing its molar mass by the number of moles of electrons transferred in a reaction. In the case of Mohr's salt, the iron (Fe²⁺) is oxidized to Fe³⁺ when it reacts with KMnO4 in an acidic medium.

Oxidation Reaction

In acidic conditions, KMnO4 acts as a strong oxidizing agent. The half-reaction for the oxidation of Fe²⁺ to Fe³⁺ can be represented as:

• Fe²⁺ → Fe³⁺ + e⁻

Here, one mole of Fe²⁺ loses one electron. Therefore, for each mole of Mohr's salt, which contains one mole of Fe²⁺, one mole of electrons is transferred during the reaction.

Calculating Equivalent Mass

Now, we can calculate the equivalent mass of Mohr's salt using the formula:

Equivalent Mass = Molar Mass / n

Where n is the number of moles of electrons transferred. Since we established that one mole of Fe²⁺ loses one electron, n is 1.

Substituting the values, we get:

Equivalent Mass = 392 g/mol / 1 = 392 g/equiv

Final Answer

Thus, the equivalent mass of Mohr's salt is 392 g/equiv. Therefore, the correct option is (A) 392.

Summary

In summary, when Mohr's salt is oxidized by KMnO4 in an acidic medium, the iron present in it undergoes a change that allows us to calculate its equivalent mass based on the number of electrons transferred. Since one mole of Fe²⁺ loses one electron, the equivalent mass remains the same as the molar mass, which is 392 g. This understanding is crucial for stoichiometric calculations in redox reactions.