Which quantity is defined as moles of solute per kilogram of solvent?

• A. Molarity
• B. Molality
• C. Normality
• D. Mass percent

Answer: (B)

Molality is defined as moles of solute per kilogram of solvent. This quantity uses the mass of the solvent in kilograms, so it stays constant with temperature changes (volume can expand or contract with temperature, but mass does not). That makes molality especially reliable for describing concentrations in reactions and when applying colligative properties, where only the amount of solute relative to solvent matters rather than how much the solution expands or contracts with temperature.

Molarity, by contrast, is moles of solute per liter of solution, which can change with temperature as the solution expands or contracts. Normality depends on the reaction and uses equivalents per liter, so it can vary with how an equivalent is defined for a given process. Mass percent expresses the ratio of solute mass to the total mass of the solution (times 100), not per kilogram of solvent, so it describes composition differently.