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

• A. Molarity
• B. Molality
• C. Normality
• D. Osmolarity

Answer: (B)

The concept being tested is the definition of molality: moles of solute per kilogram of solvent. This concentration measure uses the mass of the solvent, not the volume of the solution, so it stays essentially constant with temperature since mass doesn’t change appreciably while volume can expand or contract. That temperature stability is why molality is especially useful in calculating colligative properties like boiling-point elevation and freezing-point depression. In contrast, molarity uses liters of solution and varies with temperature, normality uses equivalents per liter, and osmolarity uses liters of solution as well, focusing on particles per volume rather than mass. So, when you have the amount of solute in moles and the mass of the solvent in kilograms, dividing gives you the molality in mol/kg.