Chemistry · Solutions and Colligative Properties

NEET (UG) 2025 — Question 75

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 ?

Option A:
The solution shows positive deviation.
Option B:
The solution shows negative deviation.
Correct
Option C:
The solution is ideal.
Option D:
The solution has volume greater than the sum of individual volumes.

Answer: B

Step-by-step solution

Mole fraction of X: $x_X = \frac{5}{5+10} = \frac{1}{3}$ . Mole fraction of Y: $x_Y = \frac{10}{15} = \frac{2}{3}$ . Expected vapour pressure (Raoult's law): $P_{calc} = P_X^0 x_X + P_Y^0 x_Y = 63 \times \frac{1}{3} + 78 \times \frac{2}{3} = 21 + 52 = 73\text{ torr}$ . Observed vapour pressure: $P_{obs} = 70\text{ torr}$ . Since $P_{obs} < P_{calc}$ , the solution shows negative deviation from Raoult's law. Thus the correct option is B.

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Exam: NEET (UG) 2025
Paper: 2025 paper
Subject: Chemistry
Chapter: Solutions and Colligative Properties
Topic: Liquid in Liquid Solutions (Raoult's Law)

Answer: B

Step-by-step solution

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