Q.62 Chemistry ⭐ 360° Option Elimination Active

NEET 2026 Chemistry Question 62 Solution

Topic: Electrochemistry · Official NTA Booklet Code 50

Question

For a salt XY, which is a strong electrolyte, the plot of Λm versus √c has a slope of −90.0 S cm² mol⁻³ᐟ² L¹ᐟ² at 298 K. At 0.01 M concentration of XY, the value of Λm is 145.0 S cm² mol⁻¹. The limiting molar conductivity of Y⁻ ion (λ°(Y⁻), in S cm² mol⁻¹) at 298 K will be (Given: λ°(X⁺) = 74.0 S cm² mol⁻¹)

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Verified Against NTA Final Answer Key

Option (1) : 80.0

📚 Electrochemistry Medium

📋 Given Parameters & Problem Setup

Salt XY Strong 1:1 electrolyte

Slope of Λ_m vs √c -90.0 S cm² mol⁻³/² L¹/²

Concentration (c) 0.01 M

Molar Conductivity (Λ_m at 0.01 M) 145.0 S cm² mol⁻¹

λ°(X⁺) 74.0 S cm² mol⁻¹

Target Limiting molar conductivity of Y⁻ ion (λ°_{Y⁻})

⚡

The 30-Second Exam Hall Shortcut: ⚡ Onsager Intercept in 3 Steps

⏱️ Save ~60 seconds

Λ_m = Λ_m° - A√c -> Add A√c to find Λ_m°, then subtract λ°(X⁺)!

1 √c = √(0.01) = 0.1.

2 Debye-Hückel-Onsager equation: Λ_m = Λ_m° - A√c, where slope = -A = -90.0 ⇒ A = 90.0.

3 Calculate Λ_m°: Λ_m° = Λ_m + A√c = 145.0 + (90.0 × 0.1) = 145.0 + 9.0 = 154.0 S cm² mol⁻¹.

4 Apply Kohlrausch's law: λ°(Y⁻) = Λ_m° - λ°(X⁺) = 154.0 - 74.0 = 80.0 S cm² mol⁻¹!

5 Select Option (1) in 25 seconds!

🛡️

Why You Can 100% Trust This Shortcut: Standard physical chemistry relationship for strong electrolytes in dilute solutions (Debye-Hückel-Onsager formulation).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Apply the Debye-Hückel-Onsager Equation

For strong electrolytes at low concentrations, molar conductivity varies linearly with the square root of concentration according to the Debye-Hückel-Onsager equation: Λ_m = Λ_m° - A√c

\Lambda_m = \Lambda_m^\circ - A \sqrt{c}

• Plot of Λ_m vs √c is a straight line with slope = -A and y-intercept = Λ_m°.
• Given slope = -90.0 S cm² mol⁻³/² L¹/² ⇒ A = 90.0 S cm² mol⁻³/² L¹/².

Step 2

Calculate the Limiting Molar Conductivity of the Salt (Λ_m°)

Substitute c = 0.01 M and Λ_m = 145.0 S cm² mol⁻¹ into the Onsager equation.

145.0 = \Lambda_m^\circ - 90.0 \times \sqrt{0.01} = \Lambda_m^\circ - 90.0 \times 0.1 = \Lambda_m^\circ - 9.0

• Λ_m° = 145.0 + 9.0 = 154.0 S cm² mol⁻¹.

Step 3

Apply Kohlrausch's Law of Independent Migration of Ions

According to Kohlrausch's Law, the limiting molar conductivity of an electrolyte can be represented as the sum of the individual contributions of the anion and cation: Λ_m°(XY) = λ°(X⁺) + λ°(Y⁻)

\lambda_{Y^-}^\circ = \Lambda_m^\circ(\text{XY}) - \lambda_{X^+}^\circ = 154.0 - 74.0 = 80.0\text{ S cm}^2\text{ mol}^{-1}

• Limiting molar conductivity of Y⁻ = 80.0 S cm² mol⁻¹.
• Matches Option (1).

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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

✓ 100% Correct

Verdict & Scientific Reason:

Mathematically exact, derived from Onsager's equation and Kohlrausch's law.

💡

Why this is airtight: None; flawless calculation.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Arises from using √c = 0.01 instead of 0.1, giving incorrect intercept shift.

⚠️

Student Fallacy (Why candidates pick this trap): Failing to take the square root of concentration c.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Simply copying the slope magnitude A = 90.0.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing the Onsager constant A with the ionic conductance.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Subtracting 9.0 instead of adding: 145 - 9 = 136, then 136 - 74 = 62 or other arithmetic inversion.

⚠️

Student Fallacy (Why candidates pick this trap): Sign error when isolating the intercept Λ_m°.

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NTA Trap Alert: Subtracting A√c Instead of Adding to Find Λ_m°

Because the equation has a minus sign (Λ_m = Λ_m° - A√c), students often mistakenly compute Λ_m° = Λ_m - A√c. Remember: limiting conductivity at zero concentration is ALWAYS higher than conductivity at non-zero concentration!

🛡️ How to avoid the -1 mark: Check intuition: As c → 0, interionic attractions disappear, so Λ_m° MUST be greater than Λ_m.

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Core NEET Syllabus Standard

NEET Chemistry Core Syllabus · Electrochemistry Standard Benchmark

Core Scientific Principle Tested:

"For strong electrolytes, Λ_m = Λ_m° - A√c, and Λ_m° = Σ ν_i λ°_i."

Syllabus Focus: Variation of conductivity and molar conductivity with concentration, Kohlrausch's law

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Onsager says: At zero dilution ions are free, add A√c to find Λ_m° key!

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NEET 2026 Chemistry Question 62 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Onsager Intercept in 3 Steps

Step-by-Step Rigorous Workflow & Mathematical Proof

Apply the Debye-Hückel-Onsager Equation

Calculate the Limiting Molar Conductivity of the Salt (Λ_m°)

Apply Kohlrausch's Law of Independent Migration of Ions

360° Option Elimination Engine Key Feature

NTA Trap Alert: Subtracting A√c Instead of Adding to Find Λ_m°

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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