Question

A solution is prepared by dissolving 2.0 g of sucrose and 2.0 g of urea in 100 g of water at 298 K. Calculate the vapour pressure of the solution, if the vapour pressure of pure water at 298 K is 23.756 torr. (Mol. wt. of urea = 60 and sucrose = 342)

Answer 1

By using Raoult’s law

`P_"solution" = P_"water"^0 xx chi_"water"`

Where:

`P_"water"^0` = 23.756 torr

`chi_"water"` ​= mole fraction of water in the solution

Molar mass of water = 18

Moles of water = `100/18` = 5.56 mol

Molar mass of sucrose = 342

Moles of sucrose = `2/342` = 0.00585 mol

Molar mass of urea = 60

Moles of urea = `2/60` = 0.0333 mol

Total moles in solution `n_"total"` = 5.56 + 0.00585 + 0.0333

`n_"total"` = 5.59915

Mole fraction of water `chi_"water" = 5.56/5.59915`

= 0.993

`P_"solution"` = 23.756 × 0.993

= 23.59 torr