' 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 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]

Solution

Chemistry ma’am · 4 steps

The moles of NaCl stay fixed, so molarity just follows the volume, and water is densest at 4 °C.

1. What stays fixed. The lid stops any water from escaping, and the solubility of NaCl does not change, so none of it crystallises out. The moles of solute are therefore constant. So M = n/V changes only through the volume V.
2. Below 4 °C. Water behaves anomalously here. It contracts on heating up to about 4 °C, where its density is maximum. Volume falls, so molarity rises.
3. Above 4 °C. Water now expands normally on heating. Volume grows, so molarity falls steadily up to 25 °C.
4. So. The plot must rise to a maximum near 4 °C and then decrease. That is the shape in (d).

The idea to keep. Molarity depends on temperature because volume does, while molality does not. With water, always think of the density maximum at 4 °C: smallest volume there means highest molarity.

Answer from the NTA final answer key. Solution by PrepWiser.