Question

'$x$' g of NaCl is added to water in a beaker with a lid. The temperature of the system is raised from $1^{\circ}C$ to $25^{\circ}C$. Which out 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]

• A.
• B.
• C.
• D.

Hint:

Density of water is maximum at 4 degrees C, so the volume is minimum and the molarity maximum there.

Answer 1

The Correct Option is D

Step 1: For a fixed mass of NaCl, molarity is inversely proportional to the volume of the solution, which follows the volume of the water.
Step 2: Water contracts on heating from 1 to 4 degrees C (its density increases) and expands after that. So the volume decreases then increases.
Step 3: Therefore the molarity increases up to 4 degrees C, passes through a maximum and then decreases.
Step 4: The curve that rises and then falls is the last one: option (4).