'x' g of NaCl is added to water in a beaker with a lid. The temperature of the system is raised from 1°C to 25°C. Which of the following plots, is best suited for the change in the molarity (M) of the solution with respect to temperature ?
[Consider the solubility of NaCl remains unchanged over the temperature range]

Molarity is given by

$$M=\frac{n}{V}$$

In this problem, a fixed mass of NaCl is added, so the number of moles of solute, $$n$$, remains constant. Since the beaker is closed, there is no loss of water due to evaporation. Therefore, only the volume of the solution changes with temperature.

Hence,

$$M\propto \frac{1}{V}$$

Thus, molarity varies inversely with the volume of the solution.

Water exhibits anomalous expansion.

• From $$0^\circ\text{C}$$ to $$4^\circ\text{C}$$, water contracts on heating, so its volume decreases.
• At $$4^\circ\text{C}$$, water attains minimum volume and maximum density.
• Above $$4^\circ\text{C}$$, water expands on heating, so its volume increases.

Since the solution is dilute, it is assumed to follow the same trend as water.

Therefore,

• From $$1^\circ\text{C}$$ to $$4^\circ\text{C}$$, volume decreases, so molarity increases.
• At $$4^\circ\text{C}$$, volume is minimum, so molarity is maximum.
• From $$4^\circ\text{C}$$ to $$25^\circ\text{C}$$, volume increases, so molarity decreases.

Hence, the graph of molarity versus temperature first increases, reaches a maximum at approximately $$4^\circ\text{C}$$, and then decreases.

Therefore, the correct graph is Option (B).