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The molar volume of liquid benzene () increases by a factor of as it vaporizes at  C and that of liquid toluene ()increases by a factor of at  C. A solution of benzene and toluene has a vapour pressure of torr. Find the mole fraction of benzene in the vapour above the solution:





Video Explanation


Let the mole fraction of benzene be .
Let the total number of moles of the solution be 1.
Hence, the number of moles of benzene are .

The molar mass of benzene is g/mol.
The mass of benzene is . The density of benzene is .

Hence, the volume of liquid benzene is .

In the vapor phase, the volume of benzene will be . The vapor pressure of benzene is 

Convert the unit of pressure from atm to torr using the relation .


The mole fraction of benzene in the vapor phase is .

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