the mole fraction of constituent A in vapour in equilibrium with binary liquid solution A and B is 3/4. if the ratio of vapour pressure of A and B at the same temperature is 3:1, what is the molefraction of A in liquid phase?

a)1/2

b)1/4

c)1

d)3/4

from Raoult''s law, the partial vapour pressure of each component in the solutionis directly proportional to its mole fraction.

so partial pressure of A= P⁰(A) * X_A = (3/4)P (let) (as mole fraction is  directly prop. to partial pressure)

similarly, partial pressure of B= P⁰(B) * XB = (1/4)P 

putting P⁰(A) =3 and P⁰(B) = 1 we get X_A = X_B =1/2 (A)