Q.73 Chemistry ⭐ 360° Option Elimination Active

NEET 2025 Chemistry Question 73 Solution

Topic: Equilibrium · Official NTA Booklet Code 45

Question

For the reaction A(g) ⇌ 2B(g), the backward reaction rate constant is higher than the forward reaction rate constant by a factor of 2500, at 1000 K. [Given: R = 0.0831 L atm mol⁻¹ K⁻¹] Kp for the reaction at 1000 K is

✓

Verified Against NTA Final Answer Key

Option (3) : 0.033

📚 Equilibrium Easy

📋 Given Parameters & Problem Setup

Reaction A(g) ⇌ 2B(g)

Temperature (T) 1000 K

Rate Constant Relation k_b = 2500 × k_f (backward rate constant is 2500 times forward)

Gas Constant (R) 0.0831 L atm mol⁻¹ K⁻¹

⚡

The 30-Second Exam Hall Shortcut: ⚡ K_c to K_p Rapid Conversion

⏱️ Save ~40 seconds

K_c = k_f / k_b = 1 / 2500. Δn_g = 2 - 1 = 1 ⟹ K_p = K_c (RT) = (1 / 2500) × (0.0831 × 1000) = 83.1 / 2500 = 0.03324 ≈ 0.033!

1 Recall definition of equilibrium constant from kinetic rates: K_c = k_forward / k_backward.

2 Given k_b = 2500 k_f ⟹ K_c = 1 / 2500 = 4 × 10⁻⁴.

3 Calculate change in gaseous moles: Δn_g = 2 - 1 = 1.

4 Relation between K_p and K_c: K_p = K_c (R T)^{Δn_g} = K_c (R T)¹.

5 Compute RT: 0.0831 × 1000 = 83.1 L atm mol⁻¹.

6 Compute K_p: K_p = 83.1 / 2500 = (83.1 × 4) / 10000 = 332.4 / 10000 = 0.03324 ≈ 0.033 atm.

7 Select Option (3) in under 15 seconds!

🛡️

Why You Can 100% Trust This Shortcut: Equilibrium Constant Thermodynamic Relationship (NCERT Chemistry · Equilibrium: K_p = K_c(RT)^(Δn_g)).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Calculate Equilibrium Constant K_c from Rate Constants

At dynamic chemical equilibrium, the rate of the forward reaction equals the rate of the backward reaction:

k_f [A] = k_b [B]^2 \implies K_c = \frac{[B]^2}{[A]} = \frac{k_f}{k_b}

• Given k_b = 2500 k_f, so:
• K_c = \frac{k_f}{2500 k_f} = \frac{1}{2500} = 4.0 \times 10^{-4}\text{ mol L}^{-1}.

Step 2

Determine Δn_g and State the K_p - K_c Equation

Calculate the difference between the number of moles of gaseous products and gaseous reactants:

\Delta n_g = n_p(g) - n_r(g) = 2 - 1 = 1

• The thermodynamic relationship linking K_p and K_c is:
• K_p = K_c (R T)^{\Delta n_g} = K_c (R T)^1.

Step 3

Calculate K_p at T = 1000 K

Substitute R = 0.0831 L atm mol⁻¹ K⁻¹, T = 1000 K, and K_c = 1/2500:

K_p = \frac{1}{2500} \times (0.0831 \times 1000) = \frac{83.1}{2500} = \frac{83.1 \times 4}{10000} = \frac{332.4}{10000} = 0.03324 \approx 0.033\text{ atm}

• Matches Option (3) 0.033.

⭐

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. This is the product RT, omitting division by 2500.

⚠️

Student Fallacy (Why candidates pick this trap): Forgetting K_c = 1/2500.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Obtained by multiplying 2500 by 83.1 (using K_c = k_b / k_f instead of k_f / k_b).

⚠️

Student Fallacy (Why candidates pick this trap): Inverting K_c definition as k_b / k_f.

3 C

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. K_p = (1/2500) × 83.1 = 0.03324 ≈ 0.033.

💡

Why this is airtight: None; exact equilibrium kinetics calculation.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Arithmetic approximation error.

⚠️

Student Fallacy (Why candidates pick this trap): Using wrong gas constant value.

⚠️

NTA Trap Alert: Inverting K_c (Multiplying by 2500 instead of Dividing)

The question states that the BACKWARD rate constant is higher than forward by 2500 (k_b = 2500 k_f). This means K_c = k_f / k_b = 1/2500. Candidates who hastily multiply 2500 × 83.1 get 2.077 × 10⁵ (Option 2)!

🛡️ How to avoid the -1 mark: Always write K_c = k_f / k_b: if backward is faster, K_c MUST be << 1!

📖

Core NEET Syllabus Standard

NEET Chemistry Core Syllabus · Equilibrium Standard Benchmark

Core Scientific Principle Tested:

"K_c = k_f / k_b and K_p = K_c (RT)^{Δn_g}."

Syllabus Focus: Relationship Between K_p and K_c

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Backward is faster ⟹ K_c = 1/2500 ⟹ K_p = 83.1 / 2500 = 0.033 (Option 3)!

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NEET 2025 Chemistry Question 73 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ K_c to K_p Rapid Conversion

Step-by-Step Rigorous Workflow & Mathematical Proof

Calculate Equilibrium Constant K_c from Rate Constants

Determine Δn_g and State the K_p - K_c Equation

Calculate K_p at T = 1000 K

360° Option Elimination Engine Key Feature

NTA Trap Alert: Inverting K_c (Multiplying by 2500 instead of Dividing)

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Equilibrium

Comparison of Molar Solubility from Solubility Product Constants (Ksp)

Direction of Reaction by Comparing Reaction Quotient (Qc) with Equilibrium Constant (Kc)

Relationship Between Kp and Kc (Δng Condition)

Equilibrium Constant and Degree of Dissociation (α) of NO(g)

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