ChemistryCBSE Class 12 · Board exam 2026, set 56/5/1 · 2 marks · Very short answer

CBSE Class 12 Chemistry 2026: question 17(a)

Set in the CBSE Class 12 board exam of 2026, paper 56/5/1, section B, question 17(a), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q17(a) · 2 marks · Very short answerSolutions

1·00 molal aqueous solution of trichloroacetic acid is heated to its boiling point. Boiling point of this solution was found to be $100.1 8^{\circ}$ C. Calculate the van’t Hoff factor for trichloroacetic acid.
(Given : $K_{b}$ for water = 0·512 K kg mol $^{- 1}$ )

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 56/5/1.

Hints

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

Hint 1

The observed $Δ T_{b}$ is the rise above 100 °C.

Hint 2

Compare the observed $Δ T_{b}$ with $K_{b} \times m$ (the value expected if $i = 1$ ), or use $Δ T_{b} = i K_{b} m$ .

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.

Observed elevation: $Δ T_{b} (obs) = 100.18 - 100 = 0.18 K$
Calculated elevation for no association or dissociation: $Δ T_{b} (cal) = K_{b} \times m = 0.512 \times 1.00$
½ mark
$Δ T_{b} (cal) = 0.512 K$
½ mark
$i = \frac{observed colligative property}{calculated colligative property} = \frac{0.18}{0.512} = 0.3516 \approx 0.35$
1 mark

Answer: $i \approx 0.35$ (0.3516)

Where the marks are

Uses $Δ T_{b} = K_{b} m$ (½ mark)
Calculated $Δ T_{b} = 0.512$ K (½ mark)
Computes $i = 0.18/0.512 = 0.3516$ (1 mark)

Revise the chapter

Chapter: Solutions (notes, formulas, where marks are lost and every past question)
Topic: Elevation of Boiling Point and Depression of Freezing Point

Practise this chapterBoard-style questions from Solutions, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 56/5/1: every question, by section.

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CBSE Class 12 Chemistry 2026: question 17(a)

Hints

Step-by-step solution

Revise the chapter

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