JEE Main 2025 Jan 24 Shift 2, Chemistry Q21: Abnormal Molar Masses

JEE Main2025Jan 24, Shift 2Chemistry

The observed and normal masses of compound $M X_{2}$ are 65.6 and 164 respectively. The percent degree of ionisation of $M X_{2}$ is ________%. (Nearest integer)

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Solution

$M X_{2}$ dissociates as: $M X_{2} \to M^{2 +} + 2 X^{-}$ , so the number of particles per formula unit is $n = 3$ .

The van't Hoff factor follows from the colligative-property ratio of molar masses:

$i = \frac{normal molar mass}{observed molar mass} = \frac{164}{65.6} = 2.5$

Relating $i$ to the degree of ionisation $α$ for one particle giving $n$ particles:

$i = 1 + (n - 1) α \Rightarrow 2.5 = 1 + (3 - 1) α = 1 + 2 α$

$\Rightarrow 2 α = 1.5 \Rightarrow α = 0.75$

$∴$ Percent ionisation $= 75%$ .

Answer: 75

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JEE Main 2025 Jan 24 Shift 2, Chemistry Q21: Abnormal Molar Masses

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