Properties such as boiling point, freezing point and vapour pressure of a pure solvent changes when solute molecules are added to get homogeneous solution. These are called colligative properties. Application of colligative properties are very useful in day-to-day life. One of its example is the use of ethylene glycol and water mixture as anti-freezing liquid in the radiator of automobiles.

A solution M is prepared by mixing ethanol and water. The mole fraction of ethanol in the mixture is 0.9.

Given : Freezing point depression constant of water (K t water ) = 1.86 K kg mol –1 Freezing point depression constant of ethanol (K b

ethanol ) = 2.0 K kg mol –1 Boiling point elevation constant of water (K b

water ) = 0.52 K kg mol –1 Boiling point elevation constant of ethanol (K b

ethanol ) = 1.2 K kg mol –1 Standard freezing point of water = 273 K

Standard freezing point of ethanol = 155.7 K

Standard boiling point of water = 373 K

Standard boiling point of ethanol = 351.5 K

Vapour pressure of pure water = 32.8 mm Hg

Vapour pressure of pure ethanol = 40 mm Hg

Molecular weight of water = 18 g mol

–1 Molecular weight of ethanol = 46 g mol

–1 In answering the following questions, consider the

olution to be ideal dilute solutions and solutes to be non volatile and non-dissociative.

(i) The freezing point of the solution M is –