Solutions: Class 12 Important Questions with Answers

Practice set: test yourself

Try every question before looking at the answers below. 64 or more out of 80 means you are ready for the board.

C. Very short answer (1 mark each)

1. Define mole fraction.
2. Give one example of a solid solution with a gas solute.
3. What is a colligative property?
4. Define ebullioscopic constant.
5. What are isotonic solutions?
6. Name the cause of bends.
7. Why is K_H larger for a less soluble gas?
8. Give an example of positive deviation.
9. Write i for a solute that dimerises completely.
10. Why is molality preferred to molarity?

D. Short answer I (2 marks each)

1. State Raoult's law and show it is a special case of Henry's law.
2. Calculate the molality of 9 g glucose in 250 g water.
3. Distinguish between positive and negative deviations (two points).
4. Why does a solution of a non-volatile solute boil at a higher temperature?
5. Find the osmotic pressure of 0.1 M glucose at 27 °C.
6. Define azeotrope; give one example of each type.
7. Why does the molar mass of acetic acid in benzene come out about 120?
8. Find ΔT_f for 0.2 m NaCl (complete dissociation).

Rough work

E. Short answer II (3 marks each)

1. 3.0 g of a non-volatile solute in 100 g water lowers the vapour pressure from 23.76 to 23.52 mm Hg. Find its molar mass.
2. 2.0 g of a solute in 50 g benzene lowers the freezing point by 0.82 K (K_f = 5.12). Find M.
3. Draw vapour pressure–composition curves for positive and negative deviations and explain.
4. Calculate π of a solution of 1.8 g glucose in 500 mL at 300 K (R = 0.0821).
5. A 0.1 m solution of a weak acid HA freezes at −0.205 °C. Find i and α (K_f = 1.86).
6. Derive the relation between relative lowering of vapour pressure and molar mass.

F. Long answer (5 marks each)

1. (a) State Henry's law and give two applications. (b) Explain bends and anoxia. (c) Find the mass of CO₂ dissolved in 1 L of water at 298 K under 2.5 atm (K_H = 1.67 × 10⁸ Pa).
2. (a) Define van't Hoff factor. (b) What is i for solutes that associate and dissociate? (c) 1.0 g of KCl in 100 g water freezes at −0.48 °C. Find i and the degree of dissociation (K_f = 1.86).

G. HOTS (higher-order thinking)

1. Why does a bottle of soda fizz when opened?
2. Why is common salt spread on roads in snowy hills, while glycol goes in car radiators?
3. Two beakers, one of water and one of sugar solution, are kept under one bell jar. What happens after a long time?
4. Why should intravenous injections be isotonic with blood?

20 MCQs

1. Unit of molality
   (a) mol L⁻¹ (b) g L⁻¹ (c) mol kg⁻¹ (d) mol
2. Temperature independent
   (a) molality (b) molarity (c) normality (d) formality
3. Henry's law: p equals
   (a) K_H/x (b) K_H + x (c) x/K_H (d) K_H x
4. Gas solubility on heating
   (a) increases (b) decreases (c) unchanged (d) doubles
5. Ideal solution
   (a) ΔH > 0 (b) ΔH = 0 (c) ΔH < 0 (d) ΔV > 0
6. Negative deviation pair
   (a) ethanol + acetone (b) hexane + heptane (c) benzene + toluene (d) chloroform + acetone
7. Minimum-boiling azeotrope from
   (a) positive deviation (b) negative deviation (c) ideal (d) none
8. Not colligative
   (a) osmotic pressure (b) ΔT_f (c) vapour pressure (d) ΔT_b
9. K_f of water
   (a) 0.52 (b) 2.53 (c) 5.12 (d) 1.86
10. i for AlCl₃ (complete)
    (a) 2 (b) 4 (c) 3 (d) 1
11. Best for molar mass of proteins
    (a) ΔT_b (b) ΔT_f (c) osmotic pressure (d) RLVP
12. RBC in hypertonic solution
    (a) shrinks (b) swells (c) bursts (d) no change
13. RLVP equals
    (a) x₁ (b) x₂ (c) p° (d) p
14. Complete dimerisation: i
    (a) 2 (b) 1 (c) 0.5 (d) 0
15. Bends is related to
    (a) Raoult's law (b) azeotrope (c) osmosis (d) Henry's law
16. Lowest freezing point (0.1 m)
    (a) K₂SO₄ (b) NaCl (c) urea (d) glucose
17. Reverse osmosis is used for
    (a) desalination (b) antifreeze (c) soda (d) welding
18. π equals
    (a) CR/T (b) RT/C (c) C/RT (d) CRT
19. Association makes molar mass
    (a) lower (b) higher (c) same (d) zero
20. H₂ in palladium is
    (a) gas in liquid (b) solid in gas (c) gas in solid (d) liquid in solid

1 (c) · 2 (a) · 3 (d) · 4 (b) · 5 (b) · 6 (d) · 7 (a) · 8 (c) · 9 (d) · 10 (b) · 11 (c) · 12 (a) · 13 (b) · 14 (c) · 15 (d) · 16 (a) · 17 (a) · 18 (d) · 19 (b) · 20 (c)

Assertion–Reason (6)

(a) Both true, R explains A. (b) Both true, R does not explain A. (c) A true, R false. (d) A false, R true.

1. A: 0.1 M NaCl has a lower freezing point than 0.1 M glucose. R: NaCl gives more particles.
2. A: Ethanol + water cannot be purified beyond about 95% by distillation. R: They form a minimum-boiling azeotrope.
3. A: Molarity of a solution changes with temperature. R: Volume of a solution changes with temperature.
4. A: Gases are more soluble in hot water. R: Dissolving a gas is exothermic.
5. A: Chloroform and acetone show positive deviation. R: They form hydrogen bonds with each other.
6. A: Acetic acid in benzene shows a higher molar mass. R: It dimerises through hydrogen bonds.

Key: 1 (a) · 2 (a) · 3 (a) · 4 (d) · 5 (d) · 6 (a)

Answers to the practice set