What is a key difference between molality and molarity?

• A. Molality uses moles of solute per kilogram of solvent; Molarity uses moles per liter of solution.
• B. Molality uses moles per liter of solution; Molarity uses grams per liter.
• C. Molality uses grams per liter; Molarity uses grams per liter.
• D. Molality uses mass fraction; Molarity uses volume fraction.

Answer: (A)

The difference hinges on what each concentration uses as its base. Molality is defined as moles of solute per kilogram of solvent, while molarity is defined as moles of solute per liter of solution. This means molality relies on the mass of the solvent (kilograms), which stays essentially constant with temperature, making it largely independent of temperature. In contrast, molarity depends on the total volume of the solution (liters), and volume can expand or contract with temperature, so molarity can change with temperature. For example, if you dissolve a certain amount of solute in a fixed kilogram of solvent, you have a specific molality; if that solution occupies a fixed liter volume, you have a specific molarity. The other phrases—grams per liter, mass or volume fractions—don’t match the standard definitions of these two concentrations.