Elements P and Q form two types of non-volatile, non-ionizable compounds PQ and PQ 2 . When 1 g of PQ is dissolved in 50 g of solvent ' A '. Δ T b was 1.176 K while when 1 g of PQ 2 is dissolved in 50 g of solvent ' A ', Δ T b was 0.689 K . ( K b of ' A ' =5 K kg mol -1 ). The molar masses of elements P and Q (in g mol -1 ) respectively, are :

Chemistry · Solutions and Colligative Properties

JEE Main 2026 — 21 January, Morning Shift — Question 55

Elements P and Q form two types of non-volatile, non-ionizable compounds PQ and $\mathrm{PQ}_{2}$ . When 1 g of PQ is dissolved in 50 g of solvent ' A '. $\Delta \mathrm{T}_{\mathrm{b}}$ was 1.176 K while when 1 g of $\mathrm{PQ}_{2}$ is dissolved in 50 g of solvent ' A ', $\Delta \mathrm{T}_{\mathrm{b}}$ was 0.689 K . ( $\mathrm{K}_{\mathrm{b}}$ of ' A ' $=5 \mathrm{~K} \mathrm{kg} \mathrm{mol}^{-1}$ ). The molar masses of elements P and Q (in $\mathrm{g} \mathrm{mol}^{-1}$ ) respectively, are :

Option A:
70110
Option B:
65145
Option C:
60,25
Option D:
25,60
Correct

Answer: D

Step-by-step solution

$\left(\Delta \mathrm{T}_{\mathrm{b}}\right)_{\mathrm{PQ}}=\mathrm{K}_{\mathrm{b}} \mathrm{m}$ $1.176=5 \times \frac{1}{\mathrm{M}_{1}} \times \frac{1000}{50}$ $\mathrm{M}_{1}=85.03$ $\left(\Delta \mathrm{T}_{\mathrm{b}}\right)_{\mathrm{PQ}_{2}}=5 \times \frac{1}{\mathrm{M}_{2}} \times \frac{1000}{50}=0.689$ $\mathrm{M}_{2}=145.13$ Let molar mass of $\mathrm{P} \& \mathrm{Q}$ are $\mathrm{M}_{\mathrm{P}}$ and $\mathrm{M}_{\mathrm{Q}}$ respectively $\mathrm{M}_{\mathrm{P}}+\mathrm{M}_{\mathrm{Q}}=85.03$ $\mathrm{M}_{\mathrm{P}}+2 \mathrm{M}_{\mathrm{Q}}=145.13$ $\mathrm{M}_{\mathrm{p}}=24.93 \approx 25$ $\mathrm{M}_{\mathrm{Q}}=60.1 \approx 60$

Answer key and solution verified before publishing.

Practise Solutions and Colligative Properties

Start with this question, then two more from the same chapter — with a tutor that explains every step. Free.

Solve a similar one free

Exam: JEE Main 2026
Paper: 21 January, Morning Shift
Subject: Chemistry
Chapter: Solutions and Colligative Properties
Topic: Abnormal Colligative Properties - van't Hoff Factor