= Molality
{title2=$b=n_{\rm solute}/m_{\rm solvent}$}
<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 $\mathrm{mol\,kg^{-1}}$. Dissolving $0.10\,\mathrm{mol}$ of solute in $0.50\,\mathrm{kg}$ of solvent produces $b=0.20\,\mathrm{mol\,kg^{-1}}$.
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.
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