Which statement distinguishes molarity and molality and indicates when one is preferred?

• A. Molarity equals moles per liter of solution; Molality equals moles per kilogram of solvent.
• B. Molarity uses mass of solvent; Molality uses volume of solution.
• C. Molality is temperature-independent and preferred for colligative properties.
• D. Molarity equals molality at standard conditions.

Answer: (C)

The key idea is how concentration measures relate to temperature and what we’re studying. Molarity is moles of solute per liter of solution, so it depends on volume. Molality is moles of solute per kilogram of solvent, so it depends on mass. Volume changes with temperature, so molarity varies with temperature, while mass stays constant, making molality effectively temperature-independent. That’s why molality is the preferred measure for colligative properties—properties that depend on the number of solute particles relative to solvent and are sensitive to temperature through volume changes. So, saying molality is temperature-independent and preferred for colligative properties captures both the distinction and its practical use. The other statements either describe definitions without addressing temperature effects or mix up what each measure uses (and the idea that molarity and molality are equal only under special conditions isn’t generally true).