1.24 g of AX 2 (molar mass 124 g mol -1 ) is dissolved in 1 kg of water to form a solution with boiling point of 100.0156 ° C , while 25.4 g of AY 2 (molar mass 250 g mol -1 ) in 2 kg of water constitutes a solution with a boiling point of 100.0260 ° C . K b ( H 2 O )=0.52 K kg mol -1 Which of the following is correct?

AX 2 is fully ionised while AY 2 is completely unionised.

AX 2 and AY 2 (both) are completely unionised.

AX 2 and AY 2 (both) are fully ionised.

AX 2 is completely unionised while AY 2 is fully ionised.

Chemistry · Solutions and Colligative Properties

JEE Main 2025 — 29 January, Morning Shift — Question 9

1.24 g of $\mathrm{AX}_{2}$ (molar mass $124 \mathrm{~g} \mathrm{~mol}^{-1}$ ) is dissolved in 1 kg of water to form a solution with boiling point of $100.0156^{\circ} \mathrm{C}$ , while 25.4 g of $\mathrm{AY}_{2}$ (molar mass $250 \mathrm{~g} \mathrm{~mol}^{-1}$ ) in 2 kg of water constitutes a solution with a boiling point of $100.0260^{\circ} \mathrm{C}$ . $\mathrm{K}_{\mathrm{b}}\left(\mathrm{H}_{2} \mathrm{O}\right)=0.52 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}$

Which of the following is correct?

Option A:
$\mathrm{AX}_{2}$ and $\mathrm{AY}_{2}$ (both) are completely unionised.
Option B:
$\mathrm{AX}_{2}$ and $\mathrm{AY}_{2}$ (both) are fully ionised.
Option C:
$\mathrm{AX}_{2}$ is completely unionised while $\mathrm{AY}_{2}$ is fully ionised.
Option D:
$\mathrm{AX}_{2}$ is fully ionised while $\mathrm{AY}_{2}$ is completely unionised.
Correct

Answer: D

Step-by-step solution

For $\mathrm{AX}_{2}:-\Delta \mathrm{T}_{\mathrm{b}}=\mathrm{K}_{\mathrm{b}} \times \mathrm{m} \times \mathrm{i}$ $0.0156=0.52 \times \frac{0.01}{1} \times \mathrm{i}_{\mathrm{AX}_{2}}$ $\Rightarrow \mathrm{i}_{\mathrm{AX}_{2}}=3 \Rightarrow$ complete ionisation For $\mathrm{AY}_{2}:-\Delta \mathrm{T}_{\mathrm{b}}=\mathrm{K}_{\mathrm{b}} \times \mathrm{m} \times \mathrm{i}$ $0.026=0.52 \times 0.0508 \times \mathrm{i}_{\mathrm{AY}_{2}}$ $\Rightarrow \mathrm{i}_{\mathrm{AY}_{2}} \simeq 1 \therefore$ complete unionisation

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Exam: JEE Main 2025
Paper: 29 January, Morning Shift
Subject: Chemistry
Chapter: Solutions and Colligative Properties
Topic: Abnormal Colligative Properties - van't Hoff Factor

If A 2 B is 30 \% ionised in an aqueous solution, then the value of van't Hoff factor (i) is × 10^-1 .

Answer: D

Step-by-step solution

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