Question

# Out of 1M urea solution and 1M KCL solution, which one has higher freezing point?

Hint: Freezing point of solution depends on Van’t Hoff Factor and it is defined as the ratio between the actual concentration of particles produced when the substance is dissolved and the concentration of a substance as calculated from its mass.

The degree of dissociation is that of the original solute molecule that has dissociated. It is mainly represented by Greek symbol $\alpha$. There is a simple relationship between this symbol and the Van’t Hoff Factor.
$\begin{gathered} i = \alpha n + (1 - \alpha ) \\ \\ \end{gathered}$
i = $1 + \alpha (n - 1)$

Freezing point depends upon Van’t Hoff Factor $(i)$
The Van’t Hoff Factor is the ratio between the actual concentration of particles produced when the substance is dissolved and the concentration of the substance as calculated from its mass.
For most non electrolytes dissolved in water the Van’t Hoff Factor is essentially 1. For most ionic compounds dissolved in water the Van’t Hoff Factor is equal to the number of discrete ions in the formula unit of the substance.
$Van't\, Hoff\, Factor\, of\, KCl\, = 2$
$Van't\, Hoff\, Factor\, of\, Urea\, = 1$
$\begin{gathered} iKCl > iUrea \\ \\ \end{gathered}$

Thus, KCL has a higher freezing point.

Note:
i) Urea serves an important role in the metabolism of nitrogen containing compounds by animals and is the main nitrogen containing compound in the urine of mammals. It is colorless, odorless, highly soluble in water and practically non toxic.
ii) KCl also known as Potassium chloride is a metal halide salt composed of potassium and chlorine. It is odorless and has a white or colorless vitreous crystal appearance.
iii) The degree of dissociation is that of the original solute molecule that has dissociated. It is mainly represented by Greek symbol $\alpha$.