A solution concentration must be expressed in molality when considering boiling-point elevation or freezing-point depression but can be expressed in molarity when consider- ing osmotic pressure. Why?

Molality (m) is defined as the number of moles of solute per kilogram of solvent, while molarity (M) is the number of moles of solute per liter of solution. Molality is temperature-independent, making it more suitable for colligative properties like boiling-point elevation and freezing-point depression, which depend on the mass of the solvent rather than the volume of the solution.

Colligative properties are physical properties of solutions that depend on the number of solute particles in a given amount of solvent, not the identity of the solute. Boiling-point elevation and freezing-point depression are examples of colligative properties, where the addition of solute alters the solvent's boiling and freezing points based on the concentration of solute particles.

Osmotic pressure is the pressure required to prevent the flow of solvent into a solution through a semipermeable membrane. It is directly proportional to the molarity of the solute in the solution, making molarity a suitable measure for osmotic pressure calculations. This relationship arises because osmotic pressure depends on the concentration of solute particles in the solution, which is effectively represented by molarity.