Question

Which of the following solutions has maximum freezing point?
(A) 1 molar of NaCl solution
(B) 1 molar KCl solution
(C) 1 molar of $CaC{{l}_{2}}$
(D) 1 molar of urea solution

Answer 1

Recollect the concept of colligative properties. Colligative properties depend only on the number of solute particles present in the solution and not on the nature of solute particles present in the solution. Think about the depression in the freezing point of each option. The solution having least depression in freezing point will have maximum freezing point.

Complete step by step answer:
- Let’s have a look at the question.
- In the question, we have to find the maximum freezing point of the solutions given in the options.
- All the four options have the same molarity, that is, all solutions are 1 molar.
- Molarity is the number of moles of solute dissolved in one litre of the solvent.
- Now, since all the solutions are one molar, let’s take a look at the compounds present in the solution and see if there is any difference in their properties.
- NaCl, KCl and $CaC{{l}_{2}}$ are ionic salts and they are electrolytes which dissociate completely in water.
- Urea is an organic covalent compound. It doesn’t dissociate in the solution. It dissolves in water due to hydrogen bonding.
- So, the number of particles of urea in 1 molar solution will be the least as compared to the number of particles in other solutions.
- Now, we know the depression in freezing point is directly proportional to molarity.
$\Delta {{T}_{f}}\propto m$
- Therefore, urea solution will have the least depression in freezing point due to less number of solute particles.
- Therefore, 1 molar urea solution will have the maximum freezing point.
So, the correct answer is “Option D”.

Note: Remember the colligative properties depend on the number of moles of solute and not on the nature of solute particles. Lower the depression in freezing point, higher will be the freezing point of a solution. Molarity is the number of moles of solute dissolved in one litre of solvent.