Question

At a given temperature vapour pressure of pure benzene is 200 atm. At the same temperature vapour pressure of the solution containing 2g of a non volatile salute in 78g of benzene is 195 atm. calculate the molar mass of solute?​

Answer 1

NAZUAK❤

The Molar mass of solute is : 12.974 g/mol.

Given, Vapour pressure of pure Benzene = 200 mm of Hg.

Mass of Solute = 2 gm, Mass of Solvent = 78 gm, Vapour Pressure = 195 mm of Hg, Molar mass of Benzene = 78 gm/mol, kb for benzene = 2.53 Kkg/mol.

We know, Depression in freezing point

ΔT_{b}T

b

= Pure solvent vapour pressure - Vapour pressure of soute added solvent.

ΔT_{b}T

b

= 200 - 195 = 5 mm of Hg.

Also,

ΔT_{b}T

b

= kb * weight of solute *1000 / Molar mass of solute * mass of solvent

5 = 2.53 * 2 * 1000 / M * 78

5 = 64.8717 / M, M = 64.8717/5 = 12.974

Therefore Molar mass of solute = 12.974 g/mol.