The vapor pressure of water at 20°C is 17.54 torr. What is the vapor pressure (in torr)of a solution containing 143.8g of dextrose (C6H12O6 ) and 146.5 g of water at ?

Question

The vapor pressure of water at 20°C is 17.54 torr. What is the vapor pressure (in torr)of a solution containing 143.8g of dextrose (C6H12O6 ) and 146.5 g of water at ?

Answer 1

This is an application of Raoult's Law which states that the vapor pressure of a solvent above a solution is equal to the mole fraction of the solvent times the vapor pressure of the pure solvent at the same temperature.

P = X_s • P⁰ where X_s is the mole fraction of the solvent, and P⁰ is the vapor pressure of the pure solvent.

#moles of dextrose = 143.8g/180.8 g/mole = 0.7954 moles.

#moles water = 146.5g / 18.02 g/mole = 8.130 moles.

mole fraction of water = 8.130/(0.7954 + 8.130) = 0.9109.

Vapor pressure of water above solution = 0.9109 x 17.54 torr = 15.98 torr.