what is the vapor pressure of water above the solution ?

Question

A solution is created by dissolving 42.6 g of glycerin C3H8O3 in 125 g of water at 343k. If the vapor pressure of pure water is 234 torr at this temperature, what is the vapor pressure of water above the solution ?

Answer 1

The vapor pressure will be a function of the mole fraction of the solute that is dissolved in the pure solvent. As the amount of solute increases, the vapor pressure of H2O decreases.

molar mass glycerin = 92.09 g/mol

molar mass H2O = 18 g/mol

moles glycerin = 42.6 g x 1 mol / 92.09 g = 0.463 mols glycerin

moles H2O = 125 g x 1 mol / 18 g = 7.11 mols H2O

Total mols = 0.463 + 7.11 = 7.57 mols

mol fraction of H2O = 7.11 / 7.57 = 0.939

Vapor pressure = 0.939 x 234 torr = 220. torr (3 sig figs)