What does molality mean?
Molality is a unit of concentration in chemistry that measures how many moles of a solute are present in one kilogram of solvent. It is especially useful when volume changes are significant, because it depends on the mass of the solvent rather than on the final volume of the solution. For example, a solution with one mole of salt dissolved in one kilogram of water has a molality of 1 mol kg⁻¹. The term is also valuable in thermodynamic calculations involving properties that depend on the amount of solvent. Although molality may sound similar to molarity, the distinction is important: molarity reports moles per litre of solution, while molality reports moles per kilogram of solvent. In everyday writing, molality is a specialist vocabulary term rather than a general alternative for concentration.
nounMolality is the concentration of a solution expressed as the number of moles of solute per kilogram of solvent. Unlike molarity, it does not depend on the total volume of the solution.
- A chemical concentration measure equal to the number of moles of solute per kilogram of solvent.
"The molality of the solution was increased by adding more solute while keeping the mass of the solvent constant."
"The chemist calculated the molality after dissolving the compound in water."
"Because the volume changed upon mixing, the solution's molality was more useful than its molarity for the calculation."
The plural is used when referring to molalities of different solutions or conditions.
"The two solutions have different molalities despite having similar concentrations by volume."
Molality measures a solution by the mass of its solvent, keeping the amount of solvent fixed even when the solution's volume changes.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does molality come from?
Molality was formed in the 20th century from the chemical unit mole together with the suffix -ity, producing a noun meaning the quality or measure of being molal. The word mole is an older English name for the gram-mole, and in its modern chemical sense it ultimately derives from Latin moles, meaning a mass or bulk. The same Latin root is reflected in related words such as molar, molecular, and molecule.
Related word forms
How molality is actually used
This is primarily a technical term in chemistry and thermodynamics. It is distinct from molarity, which uses solution volume rather than solvent mass. The symbol for molality is m, while molarity is denoted M.
Easily confused with molality
Molality is moles of solute per kilogram of solvent, whereas molarity is moles of solute per litre of the total solution.