Question

Calculate the vapor pressure of a solution prepared by dissolving 13g of n-heptane (C₇H₁₆) in 87g of n-octane (C₈H₁₈) at 25^oC). [NOTE: both heptane and octane and volatile, hence both will exert a vapor pressure when using Raoult's Law]

Answer 1

moles of n-hepante= mass/ molecular weight = 13/100 =0.13 moles of n- octane= 87/114 =0.763

Total moles = 0.763+0.13 =0.893 Mole fractions : n-heptane = 0.13/0.893=0.1456 n-octane= 0.763/0.893 =0.8544

vapor pressure of n-heptane at 25 deg.c = 46 mm Hg

Vapor pressure of n- octane at 25 deg.c =11 mm Hg

Total pressure = Mole fraction of n-heptane* pure component vapor pressure + mole fraction n-octane* pure componnent vapor pressure = 0.1456*46+0.8544*11=16.096 mm Hg

Vapor pressure in solution= mole fraction* Total pressure

Vapor pressures : n-heptane = 0.1456*16.096=2.34 mm Hg, n- octane= 0.8544*16.096=13.75 mmHg