Comprehension:
Colligative Properties
Vapour pressure of a solution decreases when a non-volatile solute is added to a volatile solvent. There are many properties of solutions which are connected with this decrease of vapour pressure. These are: (1) relative lowering of vapour pressure of the solvent (2) depression of freezing point of the solvent (3) elevation of boiling point of the solvent and (4) osmotic pressure of the solution. All these properties depend on the number of solute particles irrespective of their nature relative to the total number of particles present in the solution. Such properties are called colligative properties.
Which one of the following aqueous solutions will boil at the highest temperature?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 1m K2SO4
The elevation of boiling point is a colligative property: it depends on the number of solute particles, not on their nature. For electrolytes the van’t Hoff factor i counts the particles formed from one formula unit:
\( \displaystyle \Delta T_b=i\,K_b\,m \)
All the solutions are 1 m in water, so Kb and m are the same and the solution with the largest i boils at the highest temperature. For \( \ce{K2SO4} \):
\( \ce{K2SO4} \) → \( \ce{2K^{+}} \) + \( \ce{SO4^{2-}} \): 2 + 1 = 3 ions, so i = 3
Glucose and urea give i = 1 and KCl gives i = 2, so 3 is the largest value and 1 m K2SO4 boils at the highest temperature.