Chemistry - Vapor Pressure?

The vapor pressure of a solution containing 53.6g glycerin(C3H8O3) in133.7g ethanol (C2H5OH) is 113 torr at 40C. Calculate the vapor pressure of pure ethanol at 40.0C assuming that glycerin is a nonvolatile, nonelectrolyte solute in ethanol.

Answer:
Due to Raoult's law, the partial pressure of an substance is directly proportional to its mole fraction in the mixture.

Here, number of moles of glycerin = 53.6/92=0.5826
number of moles of ethanol = 133.7/46=2.9065

So total number of moles = 0.5826+2.9065=3.4891

Mole fraction of ethanol = 2.9065/3.4891=0.83301
When the mole fraction is this much, the pressure is 113 torr. When we are talking about pure ethanol, the mole fraction is 1.

At this mole fraction, the pressure = 133*1/0.83301=159.66 Torr.

The vapour pressure of pure ethanol is 159.66 Torr

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