What term describes the concentration of a solution expressed as moles of solute per kilogram of solvent?

• A. Molality
• B. Molarity
• C. Mass percent
• D. Mole fraction

Answer: (A)

Molality is the concentration expressed as moles of solute per kilogram of solvent. It uses the mass of the solvent, not the volume of the solution, which makes it essentially temperature-independent because mass stays the same even as temperature changes. This is why molality is especially useful in studying colligative properties like boiling-point elevation and freezing-point depression, where the effect depends on the amount of solute relative to the solvent.

This differs from molarity, which is moles per liter of solution and can vary with temperature since solution volume changes. Mass percent describes how much mass of solute is present as a percentage of the total solution mass. Mole fraction is the ratio of moles of solute to the total moles in the mixture.