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(I) On adding 100 g… (Class 12 Chemistry SQP 2026-27, Q28)

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ChemistryCBSE Class 12 · Sample paper, session 2026-27 · 3 marks · Short answer

CBSE Class 12 Chemistry sample paper 2026-27: question 28

From the official CBSE sample question paper for 2026-27, section C, question 28, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q28 · 3 marks · Short answerSolutions

(I) On adding 100 g of non-volatile solute A in 1000 g of water, the vapour gets reduced by 50 percent. Calculate the molar mass of solute A. (2)

(II) It has been observed that nitric acid solution containing 32% of water by mass, cannot be separated into its components through fractional distillation. Why? (1)

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

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

Hint 1

Relative lowering of vapour pressure equals the mole fraction of the solute.

Hint 2

A 50% drop is large, so do not use the dilute approximation; use n_B/(n_A+n_B).

Hint 3

For (II), think of mixtures that boil with constant composition.

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. (I) Relative lowering of vapour pressure:

    ½ mark

  2. mol. A 50% reduction means .

    ½ mark

  3. , so and mol.

    ½ mark

  4. g mol⁻¹

    ½ mark

  5. (II) Nitric acid with 32% water (68% HNO₃) forms a maximum-boiling azeotrope (negative deviation). Its liquid and vapour have the same composition, so it boils at constant temperature and cannot be separated by fractional distillation.

    1 mark

Answer: (I) g mol⁻¹; (II) it is an azeotrope (same composition in liquid and vapour).

Where the marks are

  • Raoult's law expression for relative lowering (½ mark)
  • Moles of water and value 0.5 (½ mark)
  • Solving gives n_B = 55.56 mol (½ mark)
  • Molar mass 1.8 g mol⁻¹ (½ mark)
  • Azeotrope with identical liquid/vapour composition (1 mark)

Revise the chapter

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 2026-27: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.