Chemistry OWLV2

Question

The vapor pressure of a solution containing 36.3 g glycerin (

CHgO3) in 118.4 g ethanol (

C,H,OH) is 138 torr at 44°C. Calculate the vapor pressure of pure ethanol at 44°C assuming that glycerin is a nonvolatile, nonelectrolyte solute in

ethanol.

Answer 1

The vapor pressure of a PURE solvent will decrease when you add a solute. The amount of that decrease is related to the mole fraction of the pure solvent. So, we need to find moles of solvent and solute and then apply Raoult's law which tells us that the vapor pressure is equal to the mol fraction x the vapor pressure of the pure solvent.

molar mass glycerin = 92.1 g / mol

molar mass ethanol = 46.1 g / mol

moles glycerin = 36.3 g x 1 mol / 92.1 g = 0.394 mols glycerin

moles ethanol = 118.4 g x 1 mol / 46.1 g = 2.568 mols ethanol

Total moles = 0.394 + 2.568 = 2.962 mols

mol fraction ethanol = 2.568 mol / 2.962 mol = 0.867

Vapor pressure of the solution = 0.867 x 138 torr = 120. torr