Why is molality used for colligative properties?
Colligative properties are physical properties of solutions, like boiling point elevation and freezing point depression. In these calculations, the temperature of the solution is changing as we add more solute to the solvent, so this means that the volume of the solution is changing. Since molarity is moles solute per liter of solution, we cannot use molarity as our concentration unit. This is why we use molality (moles solute per kg of solvent) since the kg of solvent doesn't change with temperature.