Two liquids X and Y form an ideal solution. The mixture has a vapour pressure of 400mm at 300K, when mixed in the molar ratio of 1:1 and a vapour pressure of 350mm when mixed in the molar ratio of 1:2 at the same temperature. The vapour pressures of the two pure liquids X and Y respectively are?

Using Raoult's law for the first mixture, (P_X + P_Y) / 2 equals 400, so P_X + P_Y equals 800. For the second mixture, the mole fraction of X is 1/3 and Y is 2/3, giving (P_X + 2P_Y) / 3 equals 350, so P_X + 2P_Y equals 1050. Subtracting the first equation from the second gives P_Y equals 250. Substituting back into the first equation yields P_X equals 550.

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