Q.81 Chemistry ⭐ 360° Option Elimination Active

NEET 2023 Chemistry Question 81 Solution

Topic: Electrochemistry · Official NTA Booklet Code G5

Question

The conductivity of centimolar solution of KCl at 25°C is 0.0210 ohm⁻¹ cm⁻¹ and the resistance of the cell containing the solution at 25°C is 60 ohm. The value of cell constant is -

✓

Verified Against NTA Final Answer Key

Option (4) : 1.26 cm⁻¹

📚 Electrochemistry Easy

📋 Given Parameters & Problem Setup

Conductivity (κ) 0.0210 Ω⁻¹ cm⁻¹

Resistance (R) 60 Ω

Concentration of KCl Centimolar (0.01 M)

Temperature 25°C

⚡

The 30-Second Exam Hall Shortcut: ⚡ Cell Constant Product: G* = κ × R = 0.0210 × 60 = 1.26 cm⁻¹

⏱️ Save ~40 seconds

Cell constant G* is simply the product of conductivity (κ) and resistance (R): G* = κ · R. Compute 0.0210 × 60 = 1.26 cm⁻¹ in under 5 seconds! The concentration is distractor data.

1 Recall formula: Cell constant G* = κ × R.

2 Substitute κ = 0.0210 Ω⁻¹ cm⁻¹ and R = 60 Ω.

3 Multiply: 0.0210 × 60 = 1.26 cm⁻¹.

4 Match Option (4) directly.

🛡️

Why You Can 100% Trust This Shortcut: Fundamental definition of electrolytic conductivity and cell geometry G* = l/A = κ · R.

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

State the Definition of Cell Constant

The cell constant G* represents the geometric ratio of electrode separation (l) to electrode surface area (A):

G^* = \frac{l}{A}

Step 2

Relate Cell Constant to Conductivity and Resistance

Conductivity κ (specific conductance) is defined as the reciprocal of resistivity ρ:

\kappa = \frac{1}{\rho} = \frac{1}{R} \cdot \frac{l}{A} = \frac{G^*}{R} \implies G^* = \kappa \times R

Step 3

Substitute Numerical Values

Substitute the given conductivity κ = 0.0210 Ω⁻¹ cm⁻¹ and resistance R = 60 Ω:

G^* = 0.0210\text{ }\Omega^{-1}\text{ cm}^{-1} \times 60\text{ }\Omega = 1.26\text{ cm}^{-1}

⭐

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 (1)

✗ Incorrect

Verdict & Scientific Reason:

Inverted calculation artifact from dividing resistance by incorrect factors.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate attempted to divide rather than multiply conductivity by resistance.

2 (2)

✗ Incorrect

Verdict & Scientific Reason:

Arithmetic calculation error.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate miscalculated 0.021 × 60.

3 (3)

✗ Incorrect

Verdict & Scientific Reason:

Erroneous calculation involving centimolar concentration factor.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate unnecessarily dragged concentration C = 0.01 M into a problem that only needed cell constant.

4 (4)

✓ 100% Correct

Verdict & Scientific Reason:

Accurate: G* = κ × R = 0.0210 × 60 = 1.26 cm⁻¹.

💡

Why this is airtight: None. Candidate recognized that G* is simply κ · R.

⚠️

NTA Trap Alert: Distraction by "Centimolar Solution"

The phrase "centimolar solution of KCl" is provided as realistic contextual data, but it is completely superfluous for calculating the cell constant. Students often waste time trying to compute molar conductivity Λm = (1000 · κ)/M.

🛡️ How to avoid the -1 mark: Check what is asked: Cell constant G* requires ONLY κ and R. Ignore concentration if neither Λm nor degree of dissociation is requested.

📖

Core NEET Syllabus Standard

National Eligibility cum Entrance Test (NEET UG) · Electrochemistry Standard Benchmark

Core Scientific Principle Tested:

"Cell constant G* is an intrinsic geometric property of a conductance cell given by G* = κ · R."

Syllabus Focus: Conductance in Electrolytic Solutions – Cell Constant

Preserved in Rationalized Curriculum Syllabus & Academic Standards ↗

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NEET 2023 Chemistry Question 81 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Cell Constant Product: G* = κ × R = 0.0210 × 60 = 1.26 cm⁻¹

Step-by-Step Rigorous Workflow & Mathematical Proof

State the Definition of Cell Constant

Relate Cell Constant to Conductivity and Resistance

Substitute Numerical Values

360° Option Elimination Engine Key Feature

NTA Trap Alert: Distraction by "Centimolar Solution"

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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