Molality ($b=n_{\mathrm{solute}}/m_{\mathrm{solvent}}$)

Molality is the amount of substance of a solute divided by the mass of the solvent: $b=n_{\mathrm{solute}}/m_{\mathrm{solvent}}$. Its SI unit is $\mathrm{mol}\mathrm{k}{\mathrm{g}}^{-1}$. Dissolving $0.10\mathrm{mol}$ of solute in $0.50\mathrm{kg}$ of solvent produces $b=0.20\mathrm{mol}\mathrm{k}{\mathrm{g}}^{-1}$.

The denominator contains solvent mass, rather than the volume of the resulting chemical solution. Thus molality avoids changes caused solely by thermal expansion of the chemical solution, making it useful in colligative properties and chemical thermodynamics. It must be distinguished from molar concentration, whose denominator is solution volume.