ChemistryCBSE Class 12 · Board exam 2023, set 56/1/1 · 3 marks · Short answer

CBSE Class 12 Chemistry 2023: question 26

Set in the CBSE Class 12 board exam of 2023, paper 56/1/1, section C, question 26, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q26 · 3 marks · Short answerSolutions

(a) Differentiate between Ideal solution and Non-ideal solution.
(b) 30 g of urea is dissolved in 846 g of water. Calculate the vapour pressure of water for this solution if vapour pressure of pure water at 298 K is 23·8 mm Hg.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 56/1/1.

Hints

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

Hint 1

For (a), think about Raoult's law and the enthalpy and volume change on mixing.

Hint 2

Urea is non-volatile, so use relative lowering of vapour pressure = mole fraction of solute.

Hint 3

Molar mass of urea is 60 g mol⁻¹ and of water is 18 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.

(a) An ideal solution obeys Raoult's law at all concentrations, with $Δ_{mi x} H = 0$ and $Δ_{mi x} V = 0$ . A non-ideal solution does not obey Raoult's law, with $Δ_{mi x} H \neq = 0$ and $Δ_{mi x} V \neq = 0$ (it shows positive or negative deviation).
1 mark
(b) Raoult's law for a non-volatile solute: $\frac{p _{A}^{\circ} - p _{A}}{p _{A}^{\circ}} = x_{B} = \frac{n _{B}}{n _{A} + n _{B}}$ .
½ mark
Moles of urea: $n_{B} = \frac{30}{60} = 0 \cdot 5$ mol. Moles of water: $n_{A} = \frac{846}{18} = 47$ mol.
So $1 - \frac{p _{A}}{23 \cdot 8} = \frac{0 \cdot 5}{47 + 0 \cdot 5} = \frac{0 \cdot 5}{47 \cdot 5}$ .
1 mark
$p_{A} = 23 \cdot 8 \times \frac{47}{47 \cdot 5} \approx 23 \cdot 5$ mm Hg.
½ mark

Answer: (a) Ideal obeys Raoult's law, ΔH_mix = ΔV_mix = 0; non-ideal does not. (b) p ≈ 23.5 mm Hg

Where the marks are

Any one correct difference between ideal and non-ideal solutions (1 mark)
Relative lowering of vapour pressure formula (½ mark)
Correct moles (0.5 and 47) and substitution (1 mark)
Answer ≈ 23.5 mm Hg (½ mark)

Revise the chapter

Chapter: Solutions (notes, formulas, where marks are lost and every past question)
Topic: Raoult's Law, Ideal and Non-ideal Solutions

Practise this chapterBoard-style questions from Solutions, one at a time, marked against the answer key and marking points.Back to the full paper2023 board paper, set 56/1/1: every question, by section.

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CBSE Class 12 Chemistry 2023: question 26

Hints

Step-by-step solution

Revise the chapter

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