Question

At 20 degrees Celsius the vapor pressure of benzene (C6H6) is 75 torr, and that of toluene (C7H8) is 22 torr. Assume that benzene and toluene form an ideal solution.

a) What is the composition in mole fractions of a solution that has a vapor pressure of 45 torr at 20 degrees celsius.

b) What is the mole fraction of benzene in the vapor above the solution described in part (a)?

Answer 1

a) Let us say the mole fraction of benzene is x obviously the mole fraction of toluene is 1-x.Apllying Raoult's law
\[P=P _{A}^{o} x _{a} + P _{b}^{o}x _{b}\] substitue the values and put mole fraction of b as 1-x and solve for x you'll get the answer.

Answer 2

To find mole fraction in vapour find vapour pressure of both benzene and toluene.
\[P _{C6H6}=P _{C6H6}^{o}x\] and similarly for toluene also you find the partial pressure.
Now apply dalton's law of partial pressures,
suppose P1 is pressure of Benzene and P2 is pressure of toluene then,
Mole fraction of benzene = P1/(P1+P2).