Question

In which of the following, the van’t Hoff factor(i) is equal to one?
A $NaCl$
B $KN{O_3}$
C Urea
D All of these.

Answer 1

Hint : Van’t Hoff factor is defined as the ratio of actual concentration of particle produced when it is dissolved to the concentration of a substance which is calculated from its mass. In simple words we can say $i =$ ratio of observed colligative property to the normal or theoretical colligative property.

Complete Step By Step Answer:
Van’t Hoff factor(i) is equal to the number of ions in solution.
Let's start with option A . one mole of $NaCl$ is dissolved in one litre of water and it gives one mole of $N{a^ + }$ and one mole of $C{l^ - }$ joanns. Then we consider this situation at time $t = 0$ and time equal to $t$ .
NaCl \to N{a^ + } + C{l^ - } \\\ i = 2 \\\
Similarly,
KN{O_3} \to {K^ + } + N{O^ - }_3 \\\ i = 2 \\\
Since urea is a non-electrotype, hence the Van’t Hoff factor will be equal to $1$
The correct answer is C.

Note :
In general, the value of the Van't Hoff factor for association is less than one and for dissociation it is greater than one. In case of dissociation quantity of solute and colligative property increases, molar mass of solute decreases, resulting in Van’t Hoff factor greater than one. Similarly, in case of association molar mass of solute increases, quantity of solute and colligative property value increases that is why Van’t Hoff factor less than one.