[asdfnerd7013]

Consider two liquids A and B. The vapor pressure of pure A (molecular weight = 50 g/mol) is 225 torr at 25 degrees C. The vapor pressure of pure B (molecular weight = 75 g/mol) is 90 torr at 25 degrees C.

What is the total vapor pressure at 25 degree C of a solution that is 70% A and 30% B by weight?

The answer comes out to 195 torr, and the mole fractions come out to 7/9 for A and 2/9 for B, but I'm unsure how.. Could anyone explain the steps please? Thanks!

[opti384]

What are you unsure of?

Finding the mole fraction or getting 195 torr?

[Vidya]

let the total mass of the sample = 100g
calculate mass of A =70%of 100 g
calculate moles of A = mass /molar mass
similarly do for the other part
now take out mole fractions for A and B
total vapor pressure = mole fraction of A X vapor pressure of pure A + mole fraction of B X vapor pressure of pure B
you will get the correct answer