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

Question

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.

Answer 1

Molality is used in cryoscopy and ebullioscopy for practical reasons.

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.

Answer 2

Use molality (m) for ebullioscopy and cryoscopy,as they involve temperature changes, so it would change de molarity if used (it depends on the volume). Use molarity (M) (or preferible (c) or []) for osmotic pressure and Henry's law. And finally, use mole fraction in Raoult's law, but because it's intrinsec to the formula. https://en.wikipedia.org/wiki/Colligative_properties https://en.wikipedia.org/wiki/Henry%27s_law