The two liquids hexane and heptane form a solution in which both components are volatile. Their vapor pressures at 30°C are 189 torr (hexane) and 58 torr (heptane). What is the total vapor pressure over the solution of these two liquids at 30°C in which the mole fraction of the hexane is 0.83?
Answer:
We use Raoult's law
Pa = P°a Xa
Pa = 189 x 0.83 = 156.9 torr
Since the sum of mole fractions is 1
mole fraction of the heptane = 1 - 0.83 = 0.17
Pb = P°b Xb = 58 x 0.17 = 9.86 torr
Total vapor pressure = 156.9 + 9.86 = 166.8 torr
Hexane Mol.fraction = 0.83 = 83%
83% of 189 = 0.83 x 189 = 154.4 torr
Therefore, 17% of 58 = 0.17 x 58 = 9.86 torr
Total V.P. = 164.26 torr
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