Question

At 25.00 ºC the vapor pressure of pure hexane is 151.2 torr, and that of pure heptane is 45.6 torr. What is the mole fraction of hexane in the gas phase above a solution containing 3.000 moles of hexane and 2.000 moles of heptane at 25.00 ºC?

Answer 1

The calculations are shown below.

moles fraction of hexane = moles of hexane / total moles of gases

                                        = 3 / (3+2)

                                        = 0.6

Alternatively mole fraction can also be calculated from partial pressure using the formula,

mole fraction of hexane = partial pressur of hexane / total pressure of gases

                                       = 151.2 / (151.2 + 45.6)

                                       = 0.77