Hint: Molality is a measure of the number of moles of solute in a solution corresponding to $1000g$ of solvent. Because molality is defined in terms of the mass of the solvent, not its volume, the molality of a solution does not change with temperature and Volume rises as temperature does, molarity is inversely proportional to temperature.
Complete answer:
Molality is a property of a solution and is defined as the number of moles of solute per kilogram of solvent. Molality of the solution does not change with the temperature of the solution. In case of molality it is the ratio of moles to mass. Mass is the same at any temperature hence molality won’t change with change in temperature.
Molality is used because its value does not change with changes in temperature. The volume of a solution, on the other hand, is slightly dependent upon temperature. Volume rises as temperature does, molarity is inversely proportional to temperature. As temperature increases, volume also increases, which means that it will decrease the number of moles per litre. As the temperature of a solution is increased, the average kinetic energy of the molecules that make up the solution also increases. This increase in kinetic energy allows the solvent molecules to more effectively break apart the solute molecules that are held together by intermolecular attractions.
So, the correct answer is (C) Assertion is correct but reason is incorrect.
Note:
The primary advantage of using molality as a measure of concentration is that molality only depends on the masses of solute and solvent, which are unaffected by variations in temperature and pressure. In contrast, solutions prepared volumetrically are likely to change as temperature and pressure change.
