Question

Which of the following solutions will have a lower vapour pressure and why?

(1) A 5% solution of cane sugar (C₁₂H₂₂O₁₁) 

(2) A 5% solution of urea (NH₂CONH₂)

(Relative atomic masses: H = 1, C = 12, O = 16, N = 14)

Answer 1

(1) No. of moles of cane sugar

= wt. of cane sugar / mol. wt. of cane sugar

= 5/342 

= 0.014

(2) No. of moles urea 

= wt. of urea / mol. wt. of urea

= 5/60 

= 0.083

5% urea solution will have lower vapour pressure because according to Raoult's law larger the number of moles of solute in the solution greater is the lowering of vapour pressure.