Q.81 Chemistry ⭐ 360° Option Elimination Active

NEET 2025 Chemistry Question 81 Solution

Topic: Solutions · Official NTA Booklet Code 45

Question

5 moles of liquid X and 10 moles of liquid Y make a solution having a vapour pressure of 70 torr. The vapour pressures of pure X and Y are 63 torr and 78 torr respectively. Which of the following is true regarding the described solution?

(1)The solution shows positive deviation.
(2)The solution shows negative deviation.
(3)The solution is ideal.
(4)The solution has volume greater than the sum of individual volumes.

✓

Verified Against NTA Final Answer Key

Option (2) : The solution shows negative deviation.

📚 Solutions Easy

📋 Given Parameters & Problem Setup

Moles of Liquid X (n_X) 5 moles

Moles of Liquid Y (n_Y) 10 moles

Vapor Pressure of Pure X (P_X°) 63 torr

Vapor Pressure of Pure Y (P_Y°) 78 torr

Observed Vapor Pressure (P_{obs}) 70 torr

⚡

The 30-Second Exam Hall Shortcut: ⚡ Observed vs. Raoult Expected Pressure Comparison

⏱️ Save ~30 seconds

x_X = 5/15 = 1/3, x_Y = 10/15 = 2/3. P_ideal = (1/3)(63) + (2/3)(78) = 21 + 52 = 73 torr! Observed is 70 torr < 73 torr expected ⟹ NEGATIVE DEVIATION!

1 Calculate mole fractions: x_X = 5 / (5 + 10) = 5/15 = 1/3; x_Y = 10 / 15 = 2/3.

2 Calculate expected ideal vapor pressure using Raoult’s Law:

3 P_total(ideal) = x_X P_X° + x_Y P_Y° = (1/3 × 63) + (2/3 × 78) = 21 + 52 = 73 torr.

4 Compare with experimentally measured vapor pressure: P_obs = 70 torr.

5 Since P_obs (70 torr) < P_ideal (73 torr), attractive forces between X and Y are stronger than pure liquid attractions.

6 Therefore, the solution exhibits NEGATIVE DEVIATION from Raoult’s law!

7 Select Option (2) in under 15 seconds!

🛡️

Why You Can 100% Trust This Shortcut: Raoult’s Law for Ideal Solutions (NCERT Chemistry · Solutions: P_total = P°_A x_A + P°_B x_B).

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Calculate Mole Fractions of Components X and Y

Total moles in the binary liquid solution:

n_{total} = n_X + n_Y = 5 + 10 = 15\text{ moles}

• x_X = 5 / 15 = 1/3.
• x_Y = 10 / 15 = 2/3.

Step 2

Calculate Theoretical Ideal Vapor Pressure by Raoult’s Law

If the solution were ideal, the total vapor pressure would follow Raoult’s law:

P_{ideal} = x_X P_X^\circ + x_Y P_Y^\circ = \left(\frac{1}{3} \times 63\text{ torr}\right) + \left(\frac{2}{3} \times 78\text{ torr}\right) = 21 + 52 = 73\text{ torr}

• The ideal expected total pressure is 73 torr.

Step 3

Compare Observed and Ideal Vapor Pressure

Evaluate the direction of deviation:

P_{observed} = 70\text{ torr} < P_{ideal} = 73\text{ torr}

• Because the observed vapor pressure is lower than predicted by Raoult’s law, escaping tendency is suppressed due to stronger intermolecular attractions (X-Y > X-X, Y-Y).
• This defines NEGATIVE DEVIATION from Raoult’s law, matching Option (2).

⭐

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. Positive deviation requires P_obs > P_ideal (here 70 < 73).

⚠️

Student Fallacy (Why candidates pick this trap): Thinking lower pressure means positive deviation.

2 B

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. P_obs (70 torr) is less than the calculated ideal pressure (73 torr).

💡

Why this is airtight: None; rigorous Raoult’s law computation.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. An ideal solution would exert exactly 73 torr, not 70 torr.

⚠️

Student Fallacy (Why candidates pick this trap): Rounding 70 and 73 as equal.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Solutions with negative deviation exhibit contraction in volume (ΔV_{mix} < 0).

⚠️

Student Fallacy (Why candidates pick this trap): Confusing positive deviation volume expansion with negative deviation.

⚠️

NTA Trap Alert: Confusing Positive and Negative Deviation Pressure Criteria

P_obs < P_ideal means the vapor pressure is LOWER than expected, which is NEGATIVE deviation (ΔP < 0, ΔH_{mix} < 0, ΔV_{mix} < 0). Option 4 describes positive deviation (ΔV_{mix} > 0), which is the opposite!

🛡️ How to avoid the -1 mark: Remember: Observed pressure LESS than Raoult = NEGATIVE deviation!

📖

Core NEET Syllabus Standard

NEET Chemistry Core Syllabus · Solutions Standard Benchmark

Core Scientific Principle Tested:

"P_{obs} < P_{ideal} signifies negative deviation from Raoult’s law."

Syllabus Focus: Non-Ideal Solutions: Positive and Negative Deviations from Raoult’s Law

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Expected 73, got 70 ⟹ Pressure DROPPED ⟹ Negative deviation (Option 2)!

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NEET 2025 Chemistry Question 81 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Observed vs. Raoult Expected Pressure Comparison

Step-by-Step Rigorous Workflow & Mathematical Proof

Calculate Mole Fractions of Components X and Y

Calculate Theoretical Ideal Vapor Pressure by Raoult’s Law

Compare Observed and Ideal Vapor Pressure

360° Option Elimination Engine Key Feature

NTA Trap Alert: Confusing Positive and Negative Deviation Pressure Criteria

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Solutions

Thermodynamics & Intermolecular Forces of Ideal Solutions

Raoult's Law & Binary Liquid Mixtures Deviations

Henry’s Law Constant and Gas Solubility in Liquids

Determination of Temperature from Osmotic Pressure Slope

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