A mixture of 0.2 moles of alcohol A and 0.5 moles of alcohol B has a total vapour pressure of 40 mmHg at 297 K. If the mixture obeys Raoult's law, find the vapour pressure of pure B at 297 K given that the pressure of a pure A is 20 mmHg at 297 K.

Answer 1

Calculate X for A(which I'm calling a) and X for B (which I'm calling b). Poa and Pob are P^o_a and P^o_b respectively.
pa + pb = 40 mm
pa = Xa*Poa
Knowing pa you calculate pb
Then pb = Xb*Pob
You know pb and Xb, solve for pob.
Post your work if you get stuck and I can help you through it.