Properties such as boiling point, freezing point and vapour pressure of a pure solvent change when solute molecules are added to get homogeneous solution. These are called colligative properties. Applications of colligative properties are very useful in day-to-day life. one of its examples is the use of ethylene glycol adn water mixtures 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_(f)^(water)) = 1.86 K kg mol^(-1)`
Freezing point depression constant of ethanol `(K_(f)^("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 solutions to be ideal dilute solutions and solutes to be non-volatile and non-dissociative.
The vapour pressure of the solution M is

Options

A. `39.3 mm Hg`
B. `36.0 mm Hg`
C. `29.5 mm Hg`
D. `28.8 mm Hg`

Answer

B

Explanation

`P^(s) = (P^(@))_(solvent). X_(solvent)`
`= 40 xx 0.9`
`= 36.0 mm Hg`