A solution of benzene and toluene at 25 °C exerts a total vapor pressure of 66.6 torr. What is the mole fraction of benzene in the vapor phase? The vapor pressures of pure benzene and toluene at 25 °C are 94.2 torr and 28.4 torr, respectively. 0

0.5805.

Explanation:

Using Raoult's Law;

Psol = Xbenzene * P°benzene + Xtoluene * P°toluene

where,

Psol = vapor pressure of the solution

Xbenzene = mole fraction of benzene

P°benzene = vapor pressure of pure benzene

Xtoluene = mole fraction of toluene

P°toluene = vapor pressure of pure toluene

Mole fraction is defined as the ratio between the number of moles of a component in a solution to the total number of moles in the solution.

Xbenzene + Xtoluene = 1

Let,

Y = mole fraction of toluene

X = mole fraction of benzene

X = 1 − Y

Psol = X * P°benzene + (1 − X) * P°toluene

66.6 = X * 94.2 + (1 − X) * 28.4

X = 0.5805

Therefore,

Y = 1 - 0.5805

= 0.4195

Mole fraction of benzene = 0.5805.