Which term describes solute concentration as moles per kilogram of solvent?

• A. Molarity
• B. Equivalents per kilogram
• C. Normality
• D. Molality

Answer: (D)

Molality is defined as moles of solute per kilogram of solvent. This uses the mass of the solvent, not the volume of the solution, so it stays essentially unchanged with temperature because mass doesn’t change while volumes can expand or contract. That temperature independence makes molality particularly useful for calculating colligative properties like freezing-point depression and boiling-point elevation, where the effect depends on the amount of solute particles rather than how much solution you have by volume.

Molarity, by contrast, is moles per liter of solution and can vary with temperature because solution volume changes with heating or cooling. Normality is similar but uses equivalents per liter of solution, which depends on the reaction context. Equivalents per kilogram isn’t the standard way to express concentration for most solute measurements. So the term that fits moles per kilogram of solvent is molality.