Question

Water has a vapor pressure of 47.1 torr at 37 ^oC. What is the vapor pressure of water over a solution that is made by dissolving 50.0 g of glycerin, C₃H₈O₃, in 100.0 mL of water at 37 ^oC? The density of water at 37 ^oC is 0.993 g/mL.

Please show all steps clearly :)

Answer 1

Solution:

Determination of number of moles:

Mass of water = Volume x Density

= 100 mL x 0.993 g mL-1 = 99.3 g

The number of moles of solute (glycerine)

= Mass /Molar mass = 50 g / 92.11 g/mol

= 0.5428 mol

Number of moles of water = mass /molar mass

= 99.3 g / 18.015 g mol-1 = 5.512 mol

Mole fraction of water calculation:

Mole fraction of water = number of moles of water/ Total number of moles

= 5.512 mol / (5.512 mol + 0.5428 mol) = 0.91
Vapor pressure of water over solution= Mole fraction of water x Vapor pressure of water

P = 47.1 mmHg x 0.91 = 42.86 mmHg