Please give me a few hints as to how I can approach this problem

A mixture of two miscible volatile ideal liquids P and Q (obeying Raoult’s law) is kept in a vessel (molar ratio of P and Q in the mixture is m). At a suitable temperature T, the vapour above the liquid is condensed in another vessel. The liquid obtained on condensation is allowed to evaporate and establish equilibrium with its vapour. The vapour is then condensed in another vessel. The process of such evaporation and condensation is repeated for n times. If the ratio of the vapour pressure of pure P to that of pure Q is p, the molar ratio of P and Q in the condensed liquid obtained after nth cycle (for finite n > 1) is..?

In each condensation-evaporation cycle, shouldn't the ratios of moles be the same?


1 Answer 1



The vapor pressure of a particular component in a solution is:

(Vapor pressure of that pure substance)*(Its mole fraction in solution)

Use this to find the ratio of the vapor pressure of P and Q above the initial solution. This ratio is equal to the ratio of their moles in the liquid which is obtained by condensing. Repeat n times.


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