Consider a liquid mixture of two liquid components A and B having vapour pressures \(P_1^0\) and \(P_2^0\) and mole fractions x1 and x2 respectively.
By Raoult’s law,
The vapour pressures P1 and P2 are,
P1 = x1\(P_1^0\) and P2 = x2\(P_2^0\)
The vapour pressure of the solution is,
PT = P1 + P2
= x1\(P_1^0\) + x2\(P_2^0\)
∴ PT = (\(P_2^0\) - \(P_1^0\))x2 + \(P_1^0\)
The plot of PT versus x2 is a straight line. The plots of P1 versus x1 and P2 versus x2 are straight lines passing through the origin.
When x1 = 1, x2 = 0, PT = \(P_1^0\) and when x1 = 0,x2 = 1,PT = \(P_2^0\) as shown by lines I and II in fig.
