Molal

Molality, symbol m, is a unit of concentration defined as the amount of solute in moles per kilogram of solvent. It is calculated as m = n_solute / m_solvent, where n_solute is the number of moles of dissolved substance and m_solvent is the mass of the solvent in kilograms. The solvent mass refers to the portion of the mixture that acts as the dissolving medium and is the mass used in the calculation regardless of how much solute is added. The total mass of the solution increases with solute, but molality remains based on the solvent’s mass.

Molality is expressed in units of mole per kilogram (mol/kg) and is temperature-independent in common practice, because mass is largely unaffected by temperature changes. This makes molality especially useful in calculations involving colligative properties, such as boiling-point elevation, freezing-point depression, and osmotic pressure, where the effect depends on the number of dissolved particles rather than on the solution’s volume.

By contrast, molarity (M) is moles of solute per liter of solution and can vary with temperature due to volume changes. In dilute solutions, molarity and molality are related approximately, but exact relationships require knowledge of solution density and composition.

Examples: dissolving 1 mole of solute in 1 kilogram of solvent yields a molality of 1 mol/kg. Dissolving 0.5 mole in 0.5 kilogram of solvent also yields 1 mol/kg. Molality remains a convenient measure when temperature stability and solvent quantity are central to the calculation.