Why Chemical Equation is Used to Find Molecularity and Strength of KMnO4

Why Chemical Equation is Used to Find Molecularity and Strength of KMnO₄ Using M/20 Mohr’s Salt Solution

In volumetric analysis, especially in Class 11 and Class 12 chemistry practicals, potassium permanganate (KMnO₄) is commonly standardized using M/20 Mohr’s salt solution. A very common question asked in exams is:

“Why is a chemical equation required to find the molecularity and strength of KMnO₄?”

The answer lies in the stoichiometry of the reaction and the nature of KMnO₄.

---

1. Importance of Chemical Equation in Volumetric Analysis

In titration calculations, we do not depend only on the volume of solutions. All calculations are based on the balanced chemical equation.

KMnO₄ does not react with Mohr’s salt in a simple 1:1 ratio. Therefore, without a chemical equation:

• The reaction ratio cannot be known
• The number of electrons transferred cannot be determined
• The equivalent weight and n-factor cannot be calculated

Hence, a chemical equation is compulsory to calculate the molecularity and strength of KMnO₄.

---

2. Why Mohr’s Salt is Used

Mohr’s salt (FeSO₄·(NH₄)₂SO₄·6H₂O) is used because:

• It is a primary standard
• It has high purity
• It has stable composition
• Its molarity (M/20) is accurately known

KMnO₄ is not a primary standard because it decomposes on standing and may contain impurities. Therefore, KMnO₄ solution must be standardized using Mohr’s salt.

---

3. Chemical Equation of the Reaction

The titration is carried out in acidic medium using dilute sulphuric acid. The balanced ionic equation is:

MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O

From the above equation, we observe that:

• 1 mole of KMnO₄ reacts with 5 moles of Fe²⁺
• KMnO₄ acts as an oxidizing agent
• Mohr’s salt acts as a reducing agent

---

4. Determination of Molecularity (Molarity / Normality)

From the chemical equation, it is clear that one mole of KMnO₄ gains five electrons. Therefore:

n-factor of KMnO₄ = 5

Using the titration formula:

N₁V₁ = N₂V₂

The normality of Mohr’s salt is known, and the volumes are measured experimentally. Using this equation, the normality of KMnO₄ is calculated.

Molarity of KMnO₄ is then obtained using:

M = N / n-factor

---

5. Determination of Strength of KMnO₄

Once the molarity of KMnO₄ is known, its strength is calculated by:

Strength (g/L) = Molarity × Molar Mass

Molar mass of KMnO₄ = 158 g mol⁻¹

Thus, the strength of KMnO₄ solution can be accurately determined.

---

6. Why Chemical Equation is Essential (Exam Point)

• To know the exact reaction ratio
• To calculate the n-factor of KMnO₄
• To determine molarity and strength correctly
• Because the reaction is not 1:1

---

7. One-Line Board Answer

Chemical equation is used because KMnO₄ reacts with Mohr’s salt in a fixed stoichiometric ratio, which is necessary to calculate the molecularity and strength of KMnO₄.

---

Prepared for educational and examination purposes.