Question

A solution is prepared by dissolving 396 g of sucrose (C12H22O11) in 604 g of water. What is the vapor pressure of this solution at 30° C? (The vapor pressure of water is 31.8 mmHg at 30° C.)

Answer 1

Vapor pressure of water = 31.8 mm Hg

Moles of sucrose = mass / molar mass

                          = 396 g/ 342.3 g/mol

                          = 1.16 moles

Mass of water = 642 g

Moles = 642 g / 18.02 g/mol

          = 35.63 moles

The mole fraction of water = moles of water / total moles

                                   = 35.63 moles / (35.63 + 1.16) moles

                                   = 0.968

The vapor pressure of solution with a non-volatile solute

= Pure vapor pressure * mole fraction of solvent

= 31.8 mm Hg * 0.968

= 30.78 mm Hg

=30.8 mm Hg