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Molality (m)

Quantity: molality · Category: Concentration

Molality is the amount of solute per kilogram of solvent: moles of solute divided by the mass of the solvent in kilograms. Unlike molarity, molality is temperature-independent because mass does not change with temperature, making it the standard unit for colligative-property calculations (freezing-point depression, boiling-point elevation). Its symbol is a lowercase m — do not confuse it with molarity's capital M.

Relation to SI

1 m = 1 mol/kg of solvent

Examples

  • 1.0 m sucrose = 1 mol of sucrose per 1 kg of water
  • Freezing-point depression: ΔT_f = K_f · m
  • A 1.0 M solution is NOT a 1.0 m solution — molarity is per liter of solution, molality is per kilogram of solvent