Question

a solution contains naphthalen (C10H8) dissolved in hexane (C6H14) at a concentration of 12.51% by mass. calculate the vapor pressure at 25 degrees C of hexane above the solution. The vapor pressure of pure hexane at 25 degrees C is 151 torr.

Answer 1

The vapour pressure of pure hexane = 151 Torr

12.51% by mass

Which means 12.51 g of naphthalene in 100 g of mixture
mass hexane = 100 - 12.51 = 87.49 g
So we can calculate the moles of hexane = Mass / molecular weight
moles hexane = 87.49 g/86 g/mol= 1.017
moles naphthalene = 12.51 g /128 g/mol= 0.0977
Mole fraction of hexane = moles of hexane / total moles
mole fraction hexane = 1.017 / 1.017 + 0.0977 = 0.912

So pressure = mole fraction X pressure of pure hexane = 0.912 X 151 = 137.71 torr