Ideal mixture of two miscible liquids `A` and `B` is placed in a cylinder containing piston. Piston is pulled out isothermally so that volume of liquid decreases but that of vapours increases. When negligibly small amount of liquid was left, the mole fraction of `A` in vapour phsae is `0.4`. If `P_(A)^(circ) = 0.4 atm` and `P_(B)^(circ) = 1.2 atm` at the experimental temperature, calculate the total pressure at which the liquid is almost evaporated.
