# How to remember what measure of concentration to use for colligative properties

Like the title says, how do I remember what measure of concentration(molality, molarity, mole fraction) to use for various colligative properties such as increase in boiling point when a non-volatile solute is added to a solvent or decrease in the freezing point for the same situation, osmotic pressure, decrease in vapour pressure.

I need to remember these for their use in their formulas and I wondered if someone could give a reason for the use of different measures of concentration in each of the properties, maybe I could end up remembering the logic and thus the measure of concentration.

In cryoscopy a sample of salt (or any other solute) is weighed and added to a pre-weighed amount of ice at $$0°$$C. The temperature change is measured, without taking care of the volume variation. It would have been much more difficult to do if you had wanted to obtain a given final volume. As you know, the masses are additives, but the volumes are not additive. Adding $$\pu{V_{1}}$$ mL of solute into $$\pu{V_{2}}$$ mL of solvent does not give a final volume equal to $$\pu{V_{1} + V_{2}}$$. In ebullioscopy, it is the same reasoning.