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(i) The vapour pressures… (Class 12 Chemistry 2024, Q31b)

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ChemistryCBSE Class 12 · Board exam 2024, set 56/1/1 · 5 marks · Long answer

CBSE Class 12 Chemistry 2024: question 31(b)

Set in the CBSE Class 12 board exam of 2024, paper 56/1/1, section E, question 31(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q31(b) · 5 marks · Long answerSolutions

(i) The vapour pressures of A and B at 25ºC are 75 mm Hg and 25 mm Hg, respectively. If A and B are mixed such that the mole fraction of A in the mixture is 0·4, then calculate the mole fraction of B in vapour phase.
(ii) Define colligative property. Which colligative property is preferred for the molar mass determination of macromolecules ?
(iii) Why are equimolar solutions of sodium chloride and glucose not isotonic ?

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 56/1/1.

Hints

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

Hint 1

Use Raoult's law for each component, then Dalton's law for the vapour composition.

Hint 2

For (iii), compare the van't Hoff factors in π = iCRT.

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) . By Raoult's law: mm Hg; mm Hg; mm Hg

    1 mark

  2. Mole fraction of B in vapour (Dalton's law):

    1 mark

  3. (ii) A colligative property is one that depends only on the number of solute particles in a given amount of solution, not on their nature.

    1 mark

  4. Osmotic pressure is preferred for macromolecules (measured at room temperature, large value even for dilute solutions).

    1 mark

  5. (iii) NaCl dissociates in water into Na⁺ and Cl⁻ (i ≈ 2), whereas glucose does not dissociate (i = 1). Since , the NaCl solution has nearly double the osmotic pressure, so equimolar solutions are not isotonic.

    1 mark

Answer: (i) y_B = 0.33; (ii) property depending on number of solute particles; osmotic pressure; (iii) NaCl dissociates (i = 2), glucose does not (i = 1)

Where the marks are

  • Partial pressures and P_total = 45 mm Hg (1 mark)
  • y_B = 15/45 = 0.33 (1 mark)
  • Definition of colligative property (1 mark)
  • Osmotic pressure (1 mark)
  • NaCl dissociates (i = 2) while glucose does not, so different π (1 mark)

Revise the chapter

  • Chapter: Solutions (notes, formulas, where marks are lost and every past question)
Practise this chapterBoard-style questions from Solutions, one at a time, marked against the answer key and marking points.Back to the full paper2024 board paper, set 56/1/1: 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.