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A. While giving… (Class 12 Chemistry SQP 2024-25, Q33b)

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ChemistryCBSE Class 12 · Sample paper, session 2024-25 · 5 marks · Long answer

CBSE Class 12 Chemistry sample paper 2024-25: question 33(b)

From the official CBSE sample question paper for 2024-25, section E, question 33(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q33(b) · 5 marks · Long answerCompetency-basedSolutions

a. While giving intravenous injections to the patients, the doctors take utmost care of the concentration of the solution used. Why is it necessary to check the concentration of the solution? (2)
b. A solution of phenol was obtained by dissolving kg of phenol in 1 kg of benzene. Experimentally it was found to be 73 % associated. Calculate the depression in the freezing point recorded. (3)

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

Compare the osmotic pressure of the injected fluid with that of the fluid inside blood cells. Which way does water move when they differ?

Hint 2

Phenol forms dimers in benzene. How does association change the number of particles, and hence the van't Hoff factor i?

Hint 3

Find i from α = 0.73 and n = 2, then use ΔTf = iKf m with Kf of benzene = 5.12 K kg mol⁻¹ and M(phenol) = 94 g mol⁻¹.

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. a. An intravenous fluid must be isotonic with blood, i.e. have the same osmotic pressure as the fluid inside the blood cells (about 0.9% saline). Only then is there no net movement of water across the cell membranes, and the cells keep their normal shape.

    1 mark
    • If the solution is more concentrated (hypertonic), water flows out of the red blood cells by osmosis, and the cells shrink.
    • If it is less concentrated (hypotonic), water flows into the cells, which swell and may burst.

    Both situations can be life-threatening, so the concentration must be checked.

    1 mark

  3. b. Phenol dimerises in benzene: .

    For association into n-mers, , with and .

    ½ mark

  4. ½ mark

  5. Use .

    Data: (benzene) K kg mol⁻¹, kg g, (phenol) g mol⁻¹, kg.

    ½ mark

  6. ½ mark

  7. 1 mark

Answer: a. The IV solution must be isotonic with blood; hypertonic fluid shrinks the cells, hypotonic fluid swells or bursts them. b. i = 0.635, ΔTf ≈ 0.69 K.

Where the marks are

  • States that the injected solution must be isotonic with blood (1 mark)
  • Explains that hypertonic fluid causes cell shrinkage and hypotonic fluid causes swelling or bursting (1 mark)
  • Uses the association relation i = 1 − α(1 − 1/n) with n = 2 (½ mark)
  • Gets i = 0.635 (½ mark)
  • Writes ΔTf = iKf·wB/(MB·wA) (½ mark)
  • Substitutes the values correctly (½ mark)
  • Gets ΔTf ≈ 0.69 K (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 2024-25: 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.