The dilute solutions of non-volatile solutes exhibit certain characteristic properties which do not depend upon the nature of the solute but depend only on the number of particles (molecules or ions) of the solute i.e., on the molar concentration of the solute. These are called colligative properties (colligative, from Latin: co means together, ligare means to bind). Thus, the properties of the solutions which depend only on the number of solute particles but not on the nature of the solute are called colligative properties.

The four important colligative properties are:

1. Relative lowering in vapour pressure
2. Elevation in boiling point
3. Depression in freezing point
4. Osmotic pressure

Which of the following is Raoult’s law applicable to, in order to determine molar masses correctly?

Non-ionic solute in dilute solution

The answer is 2) Non-ionic solute in dilute solution.

Raoult's law states that the partial pressure of a component in a mixture is equal to the vapor pressure of the pure component multiplied by its mole fraction in the mixture. This law is applicable to non-ionic solutes in dilute solutions because the solute molecules do not interact with each other significantly. As a result, the solute molecules behave as if they were individual particles, and Raoult's law applies.

Ionic solutes and concentrated solutions do not obey Raoult's law because the solute molecules interact with each other significantly. This interaction between the solute molecules causes the vapor pressure of the solution to be lower than predicted by Raoult's law.

Ionic solids in insoluble form in solvent do not obey Raoult's law because the solute molecules are not present in the solution. As a result, there is no vapor pressure of the solute to be measured.

Therefore, the only option that is applicable to Raoult's law in order to determine molar masses correctly is a non-ionic solute in a dilute solution.