Question

The vapor pressures of benzene and methylbenzene at 20°C are 75 Torr and 21 Torr, respectively. What is the composition of the vapor in equilibrium with a mixture in which the mole fraction of benzene is 0.75?

Answer 1

total Pressure = sum of partial pressures

Partial vapor pressure =mol fraction x Pure vapor pressure

hence, partial Pressure of benzene = 0.75 x 75 = 56.25 torr ( mole fraction of benzene is 0.75, given)

Now, mole fraction of methylbenzene in mixture = 1 - 0.75 = 0.25

therefore,

Partial Pressure of of methylbenzene= 0.25 x 21 = 5.25

total Pressure = 56.25 + 5.25 = 61.5 torr

now in vapor state mol fraction of benzene = partial P of benzene / total vapor presure

= 56.25 / 61.5 = 0.915

methylbenzene mol fraction in vapor = 1-0.915 = 0.085

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