Colligative Properties - UNSOLVED PRACTICE SET

Q7. Define colligative properties. Name the four colligative properties and state what they depend on.

Q8. Why are colligative properties best studied using dilute solutions rather than concentrated solutions?

Q9. Explain why the elevation of boiling point and depression of freezing point are considered colligative properties.

Q10. What is the van't Hoff factor? How does it modify the expressions for colligative properties when electrolytes are involved?

Q11. Your mother adds salt to water before boiling pasta. She also adds salt to ice when making ice cream. Explain both actions in terms of colligative properties.

Q12. Why is osmotic pressure considered the most suitable colligative property for determining the molar mass of proteins and polymers?

Q13. Discuss colligative properties in detail:

(a) Definition and common features of all colligative properties

(b) Relative lowering of vapour pressure — derivation and relationship to mole fraction

(c) Elevation of boiling point — derivation and relationship to molality

(d) Depression of freezing point — derivation and relationship to molality

(e) Osmotic pressure — derivation and van't Hoff equation

(f) Why these properties depend only on the number of solute particles

Q14. Explain the role of the van't Hoff factor in colligative properties:

(a) Definition of van't Hoff factor (i)

(b) For non-electrolytes: i = 1

(c) For electrolytes: i > 1 (dissociation), i < 1 (association)

(d) Relationship between i and degree of dissociation (α)

(e) Modified expressions for all colligative properties including i

Give examples: NaCl (i ≈ 2), K₂SO₄ (i ≈ 3), acetic acid in benzene (i < 1 due to dimerisation).

Q15. Colligative properties have profound applications in Indian science and daily life. Discuss:

(a) How reverse osmosis plants in Chennai and other coastal cities use osmotic pressure principles for desalination

(b) Why adding salt to roads in Kashmir and Himachal Pradesh melts ice in winter

(c) The use of boiling point elevation in determining the purity of ghee and other food products

(d) How cryoscopy (freezing point depression) is used in dairy industry to detect water adulteration in milk

Q16. A solution contains 5.0 g of a non-volatile solute in 95 g of water. The vapour pressure of pure water at 25°C is 23.76 mmHg.

(a) Calculate the relative lowering of vapour pressure.

(b) Calculate the vapour pressure of the solution.

(c) If the solute were NaCl (complete dissociation), calculate the new vapour pressure.

[Given: Molar mass of solute = 180 g/mol, NaCl = 58.5 g/mol]

Q17. The cryoscopic constant (Kf) for water is 1.86 K·kg/mol. A solution of 1.0 g of a non-electrolyte in 100 g of water freezes at -0.186°C.

(a) Calculate the molality of the solution.

(b) Calculate the molar mass of the solute.

(c) If the solute were MgCl₂ (i = 2.7), calculate the freezing point of a solution containing 1.0 g MgCl₂ in 100 g water.

[Given: Molar mass of MgCl₂ = 95 g/mol]

Q18. A reverse osmosis plant in Chennai desalinates seawater containing 35 g/L of NaCl.

(a) Calculate the osmotic pressure of seawater at 25°C (assume complete dissociation, i = 2).

(b) Calculate the minimum pressure that must be applied for reverse osmosis to occur.

(c) If the plant produces 10 million litres of fresh water per day, calculate the energy required if the pump operates at 70% efficiency and the applied pressure is 60 atm.

[Given: R = 0.0821 L·atm/K·mol, density of seawater ≈ 1.025 g/mL]