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

The 5% urea solution will have a lower vapour pressure than the 5% cane sugar solution.

Reason:
Vapour pressure lowering is a colligative property and depends on the number of solute particles (moles), not their type.

1. Molar mass of cane sugar (C₁₂H₂₂O₁₁) = 342 g/mol → moles in 5 g = `5/342` = 0.0146 mol.
2. Molar mass of urea (NH₂CONH₂) = 60 g/mol → moles in 5 g = `5/60` = 0.0833 mol.

Since urea gives more moles in the same mass, it causes greater vapour pressure lowering.