Raoult's Law and volume changes

[sgstudent]

When I have a pure liquid in an enclosed container, there is a fixed vapour pressure at a specific temperature. So if i were to increase the volume of the container, more liquid would turn into a vapour to go back to the same vapour pressure as before the expansion occurs.

However, if i had a mixture of liquids A and B. When I expand it what would happen? now P_A =X_x P_A° so P_A° is fixed but what about the PSUB]A[/SUB] ? Would it stay the same as before? Similarly for P_B would it remain the same such that the total vapour pressure remains the same?

I can't think of a reason why it should or should not remain the same though. For a pure liquid it makes sense that it should stay constant, but when it is a mixture of liquid I'm not too sure if the partial vapour pressure of the 2 liquids would remain the same after expansion. Can someone explain this to me?

Thanks for the help :)

 

[Chestermiller]

This question can be answered using modeling by doing the calculation described for a mixture that obeys Raoult's Law. Certainly, even if the temperature were held constant, the pressure would not remain constant unless the equilibrium vapor pressures of the two species were the same.