ChemistryCBSE Class 12 · Sample paper, session 2025-26 · 3 marks · Short answer

CBSE Class 12 Chemistry sample paper 2025-26: question 22

From the official CBSE sample question paper for 2025-26, section C, question 22, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q22 · 3 marks · Short answerSolutions

What will be the vapour pressure of a 1 molal aqueous solution of $MgC l_{2}$ , assuming dissociation of $MgC l_{2}$ to be 70 mole percent? (Vapor pressure of pure water at 25 $^{\circ}$ C is 23.8 mmHg)

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

Open one hint at a time and try again before the next.

Hint 1

Find i first, using α = (i − 1)/(n − 1) with the number of ions formed by MgCl₂.

Hint 2

Then use relative lowering of vapour pressure = i × mole fraction of solute, with 1000 g water = 55.5 mol.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

1 molal means 1 mol MgCl $_{2}$ in 1000 g water: $n_{B} = 1$ mol and $n_{A} = \frac{1000}{18} = 55.5$ mol.
½ mark
Dissociation: $MgCl_{2} \to Mg^{2 +} + 2 Cl^{-}$ , so $n = 3$ ions and $α = 0.7$ .
½ mark
$α = \frac{i - 1}{n - 1} \Rightarrow i = 1 + α (n - 1) = 1 + 0.7 \times 2 = 2.4$
½ mark
Raoult's law with van't Hoff factor: $\frac{P _{A}^{o} - P _{s}}{P _{A}^{o}} = i \frac{n _{B}}{n _{A} + n _{B}}$
½ mark
$\frac{23.8 - P _{s}}{23.8} = \frac{2.4 \times 1}{55.5 + 1} = \frac{2.4}{56.5} = 0.0425$
½ mark
$23.8 - P_{s} = 23.8 \times 0.0425 = 1.01 \Rightarrow P_{s} = 22.79 \approx 22.8$ mm Hg
½ mark

Answer: Vapour pressure of the solution ≈ 22.8 mm Hg

Where the marks are

Moles of solute (1) and water (55.5) (½ mark)
Dissociation equation with n = 3 (½ mark)
Calculates i = 2.4 (½ mark)
Writes relative lowering formula including i (½ mark)
Correct substitution (½ mark)
Final Ps ≈ 22.8 mm Hg (½ mark)

Revise the chapter

Chapter: Solutions (notes, formulas, where marks are lost and every past question)
Topic: Abnormal Molar Mass and van't Hoff Factor

Practise this chapterBoard-style questions from Solutions, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2025-26: every question, by section.

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CBSE Class 12 Chemistry sample paper 2025-26: question 22

Hints

Step-by-step solution

Revise the chapter

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