Benzene is a trace component of gasoline What is the mole ra

Benzene is a trace component of gasoline. What is the mole ratio of benzene to n-octane in the vapor above a solution of 20.0% benzene and 80% n-octane by mass at 25C? The vapor pressures of n-octane and benzene at 25 c are 11 torr and 95 torr, respectively

Solution

Consider 100 g of solution

Moles of benzene = mass/molar mass of benzene

= 20/78.11 = 0.25605 mol

Moles of n-octane = mass/molar mass of n-octane

= 80/114.23 = 0.70034 mol


P(benzene) = liquid mole fraction of benzene x vapor pressure of benzene

= 0.25605/(0.25605 + 0.70034) x 95 = 25.434 torr

P(n-octane) = liquid mole fraction of n-octane x vapor pressure of n-octane

= 0.70034/(0.25605 + 0.70034) x 11 = 8.055 torr


Vapor mole ratio of benzene : n-octane = P(benzene) : P(n-octane)

= 25.434 : 8.055

= 3.16 : 1 = 3.2 : 1