Question

Glycerin, C3H8O3, is a nonvolatile nonelectrolyte with a density of 1.26 g/mL at 25oC. Calculate the vapor pressure at 25oC of a solution made by adding 50.0 mL of glycerin to 500.0 mL of water. The vapor pressure of pure water at 25oC is 23.8 torr.

Answer 1

Partial pressure by solvent (when a non volatile solute is used) = $mole fraction of solvent * vapour pressure of pure solvent$

Therefore, no. of moles of water in 500mL = 500/18

= 27.78

no. of moles of glycerine =$\frac{density * volume}{Molar mass of glycerine}$

=1.26*50/92.1

= 0.684

total moles in solution = 28.464

vapour pressure of solution = $27.78*23.8/28.64$

= 23.085 Torr