Q.61 Chemistry ⭐ 360° Option Elimination Active

NEET 2025 Chemistry Question 61 Solution

Topic: Electrochemistry · Official NTA Booklet Code 45

Question

If the molar conductivity (Λm) of a 0.050 mol L⁻¹ solution of a monobasic weak acid is 90 S cm² mol⁻¹, its extent (degree) of dissociation will be [Assume Λ°₊ = 349.6 S cm² mol⁻¹ and Λ°₋ = 50.4 S cm² mol⁻¹.]

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Verified Against NTA Final Answer Key

Option (3) : 0.225

📚 Electrochemistry Easy

📋 Given Parameters & Problem Setup

Concentration (C) 0.050 mol L⁻¹

Molar Conductivity at Concentration C (Λ_m) 90 S cm² mol⁻¹

Limiting Molar Conductivity of Cation (λ₊°) 349.6 S cm² mol⁻¹

Limiting Molar Conductivity of Anion (λ₋°) 50.4 S cm² mol⁻¹

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The 30-Second Exam Hall Shortcut: ⚡ Kohlrausch Ratio Shortcut

⏱️ Save ~30 seconds

Λ_m° = 349.6 + 50.4 = 400 S cm² mol⁻¹. Degree of dissociation α = Λ_m / Λ_m° = 90 / 400 = 9 / 40 = 0.225!

1 Apply Kohlrausch’s law: Limiting molar conductivity of weak acid HA is the sum of ionic conductivities: Λ_m° = λ₊° + λ₋°.

2 Add the numbers: 349.6 + 50.4 = 400.0 S cm² mol⁻¹.

3 Degree of dissociation formula: α = Λ_m / Λ_m°.

4 Divide: α = 90 / 400 = 9 / 40 = 0.225.

5 Select Option (3) in under 15 seconds!

🛡️

Why You Can 100% Trust This Shortcut: Kohlrausch’s Law of Independent Migration of Ions (NCERT Chemistry · Electrochemistry: Λ°_m = ν₊ λ°₊ + ν₋ λ°₋).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Apply Kohlrausch’s Law of Independent Migration of Ions

For a monobasic weak acid HA dissociating into H⁺ and A⁻, the limiting molar conductivity at infinite dilution is the sum of the limiting molar conductivities of its constituent ions:

\Lambda_m^\circ = \lambda_+^\circ + \lambda_-^\circ = 349.6\text{ S}\cdot\text{cm}^2\cdot\text{mol}^{-1} + 50.4\text{ S}\cdot\text{cm}^2\cdot\text{mol}^{-1} = 400.0\text{ S}\cdot\text{cm}^2\cdot\text{mol}^{-1}

• Λ_m° represents the theoretical conductivity at 100% complete dissociation.

Step 2

Calculate Degree of Dissociation (α)

The degree of dissociation α of a weak electrolyte at concentration C is the ratio of its molar conductivity Λ_m to its limiting molar conductivity Λ_m°:

\alpha = \frac{\Lambda_m}{\Lambda_m^\circ} = \frac{90\text{ S}\cdot\text{cm}^2\cdot\text{mol}^{-1}}{400.0\text{ S}\cdot\text{cm}^2\cdot\text{mol}^{-1}} = \frac{9}{40} = 0.225

• Expressed as a percentage, α = 22.5%.

Step 3

Match with Multiple Choice Options

The calculated degree of dissociation is 0.225:

\alpha = 0.225 \implies \text{Option (3)}

• Matches Option (3) exactly.

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 A

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Dividing by 800 or arithmetic error.

⚠️

Student Fallacy (Why candidates pick this trap): Arithmetic miscalculation.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Corresponds to 50/400 = 0.125.

⚠️

Student Fallacy (Why candidates pick this trap): Using anion conductivity 50 instead of given Λ_m = 90.

3 C

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. α = 90 / (349.6 + 50.4) = 90 / 400 = 0.225.

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Why this is airtight: None; textbook electrochemistry calculation.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Subtraction error in long division.

⚠️

Student Fallacy (Why candidates pick this trap): Inaccurate decimal division.

⚠️

NTA Trap Alert: Attempting to Use Ostwald Dilution Law with Concentration C

The problem gives concentration C = 0.050 mol L⁻¹, which tempts students to set up K_a = Cα² / (1-α). But K_a is not requested—the question asks ONLY for the degree of dissociation α, which is simply Λ_m / Λ_m°!

🛡️ How to avoid the -1 mark: Concentration C = 0.050 is extraneous data here; α is directly Λ_m / Λ_m°.

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Core NEET Syllabus Standard

NEET Chemistry Core Syllabus · Electrochemistry Standard Benchmark

Core Scientific Principle Tested:

"α = Λ_m / Λ_m° where Λ_m° = Σ ν_i λ_i°."

Syllabus Focus: Kohlrausch’s Law and Applications to Weak Electrolytes

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Sum ions = 400 ⟹ α = 90 / 400 = 9/40 = 0.225!

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NEET 2025 Chemistry Question 61 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Kohlrausch Ratio Shortcut

Step-by-Step Rigorous Workflow & Mathematical Proof

Apply Kohlrausch’s Law of Independent Migration of Ions

Calculate Degree of Dissociation (α)

Match with Multiple Choice Options

360° Option Elimination Engine Key Feature

NTA Trap Alert: Attempting to Use Ostwald Dilution Law with Concentration C

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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