Two liquids A and B on mixing form an ideal solution. The vapour pressure of the solution containing 2 moles of A and 4 moles of B is 0.2 atm. If the vapour pressure of pure A at a certain temperature is 0.3 atm then what will be the vapour pressure of pure B at that temperature?

Answer 1

nA = 2 mols
nB = 4 mols
nTotal = 6 mols

XA = (2/6) = about 0.3
XB = (4/6) = about 0.7--you can do these more accurately and I've estimated the other values below, also, so you will need to redo the whole problem.
pA = XA*PoA
pA = 0.3* 0.3 = about 0.09

Ptotal = 0.2 = pA + pB
You know Ptotal and pA, solve for pB

Then pB = XB*PoB
You know pB and XB, solve for PoB.