Elevation of boiling point and depression in freezing point are proportional to molality, but Osmotic pressure is proportional to Molarity. Why they show difference in terms of concentration terms, though all the threeare colligative properties?

To answer this question, we need togo back to the definations.

molality-it is the amount of solute present per unit mass of the solvent present in a solution.

molarity-it is the amount of solute present per unit volume of the solvent present in a solution.

from the defination we can see that with change in temperature, the volume, not the mass of a solvent changes. hence, molarity changes, not molalaity. hence in the concepts involving change in temperature, like elevation in boiling point, etc it is wise to use molality as it does not changes with temperature.

Approved