CBSE Class 12 Chemistry Solutions Questions Answers

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CHAPTER 2-SOLUTIONS

1 MARK QUESTIONS

1 What is molarity?

2 What do you understand by saying that molality of a solution is 0.2?

3 Why is the vapour pressure of a liquid remains constant at constant temperature?

4 Define Azeotropes?

5 Which substance is usually added into water in the car radiator to act as antifreeze?

6 Which liquids form ideal solution?

7 Which property of solution depend only upon the number of moles of solute dissolved and not on the nature of the solute?

8 Write one example each of solid in gas and liquid in gas solution?

9 What is molal elevation constant or ebullioscopic constant?

10 Define van’t Hoff factor.

11 Two liquids A and B boil at 120 C and 160 C respectively. Which of them has higher vapour pressure at 70 C?

12 What happens when blood cells are placed in pure water?

13 What is the effect of temperature on the molality of a solution?

14 Write Henry’s law.

15 What is antifreeze?

16 Why cutting onions taken from the fridge is more comfortable than cutting onions lying at room temperature?

17 What will be the van’t Hoff factor for O.1 M ideal solution?

18 What is the optimum concentration of fluoride ions for cleaning of tooth?

19 What role does the molecular interaction play in the solution of alcohol and water?

20 Henry law constant for two gases are 21.5 and 49.5 atm, which gas is more soluble .

ANSWER KEY FOR 1 MARK

1 The number of moles of solute dissolved in one litre or 1dm3 of solution is known as molarity.

2 This means that 0.2 mol of the solute is dissolved in 1Kg of the solvent

3 At equilibrium, the rate of evaporation = rate of condensation. Hence the vapour pressure of a liquid is constant at constant temperature.

4 Constant boiling mixtures are called Azeotropes

5 Ethylene glycol is usually added into water in the car radiator to act as antifreeze.

6 Liquids having similar structure and polarities

7 Colligative properties.

8 Solid in gas e.g. Camphor in nitrogen gas. Liquid in gas – e.g. Chloroform mixed with N2 gas

9 The elevation in boiling point which takes place when molality of the solution is unity, is known as ebullioscopic or molal elevation constant.

10 The ratio of the observed colligative property to the theoretical value is called van’t Hoff factor.

11 Lower the boiling point, more volatile it is .So liquid A will have higher vapour pressure at 70 oC.

12 Water molecules move into blood cells through the cell walls. So, blood cells swell and may even burst.

13 No effect.

14 The solubility of a gas in a liquid is directly proportional to the partial pressure of the gas at a given temperature.

15 An antifreeze is a substance which is added to water to lower its freezing point. e.g. Ethylene glycol

16 The vapour pressure is low at lower temperature. So, less vapours of tear – producing chemicals are produced

17 Van't Hoff factor = 1, because ideal solution does not undergo dissociation or association.

18 The optimum concentration of fluoride ions for the cleaning of tooth is 1.5 ppm. [If it is more than 1.5 ppm it can be poisonous and if less than 1.5 ppm it ineffective.]

19 Positive deviation from ideal behavior.

20 KH is inversely proportional to solubility

SHORT ANSWER TYPE QUESTIONS OF 2 MARK

1 State Raoult’s law. Prove that it is a special case of Henry’ law?

2 List two conditions that ideal solutions must satisfy.

3 Explain ideal and non-ideal solutions with respect to intermolecular interactions in a binary solution of A and B.

4

a. What are minimum boiling and maximum boiling azeotropes?

b. Can azeotropes be separated by fractional distillation?

5

a. When a non-volatile solute is added to solvent, there is increase in boiling point of solution. Explain.

b. Define ebullioscopic constant and give its units.

6 How did Van’t Hoff explain the abnormal molecular masses of electrolytes like KCl in water and non-electrolytes like benzoic acid in benzene?

7 When a pressure higher than the osmotic pressure is applied on the surface of the solution separated from a solvent by semi permeable membrane, what will happen?

8 The freezing depression of 0.1M sodium chloride solution is nearly twice that of 0.1 M glucose solution. Explain?

9 The depression in freezing point is a colligative property. Explain.

10 Equimolar solution of glucose and Common salt are not isotonic. Why?

Answer key

1 Raoult’s law states that partial pressure of a volatile component of a solution is directly proportional to its mole fraction. It is a special case of Henry’ law because it becomes the same when “Kh” (Henry constant) is equal to pressure of pure solvent.

2 a. Hmixing and Vmixing of ideal solutions should be zero.

   b. They should obey Raoult’s law over the entire range of concentration.

3 For the given binary solution of A and B, it would be ideal if A-B interactions are equal to A-A and B-B interactions and it would be non-ideal if they are different to each other.The deviation from ideal behavior will be positive if A-B interactions are weaker as compared to A-A and B-B. The deviation will be negative if A-B interactions are stronger as compared to A-A and B-B.

4 (i) Minimum boiling azeotropes are the non-ideal solutions showing positive deviation while maximum boiling azeotropes are those which show negative deviation. Because of positive deviation their vapour pressures are comparatively higher and so they boil at lower temperatures while in case of negative deviation, the vapour pressures are lesser and so higher temperature are required for boiling them.(ii) No, azeotropes can’t be separated by fractional distillation

5 (i) When a non-volatile solute is added to a volatile solvent the vapour pressure of pure solvent decreases because a part of the surface is occupied by nonvolatile solute which can’t volatilise. As a result, the vapour pressure of solution decreases and hence, the solution requires a comparatively higher temperature to boil causing an elevation of boiling point. (ii) Ebullioscopic constant is defined as the elevation in boiling point of a solution of a non-volatile solute when its molality is unity. Its units are K Kg mol-1

6 The molecular mass of KCl in aqueous medium has been observed to be almost half than expected and it has been explained as dissociation of KCl into K+ ions and Cl- ions when actual no. of particles become double and so become the colligative properties but since molecular mass is always inversely proportional to colligative property it becomes almost half.In case of benzoic acid in benzene, association of molecules take place when they dimerise and their no. becomes almost half and so molecular mass doubles as a result.

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