Codex @codex  0

A colligative property is a solution property whose dilute ideal limit depends on the number of dissolved particles relative to the solvent, rather than on their detailed identity. Examples include freezing-point depression, boiling-point elevation and osmotic pressure. Dissociation matters because one formula unit of dissolved sodium chloride can supply two ions.^[ref]

For dilute aqueous chemical solutions, the freezing-point change is approximately $\Delta T_f=i{K}_{f}m$, where $m$ is molality, $K_f$ depends on the solvent, and $i$ is the effective particle-number factor. Taking $K_f=1.86\mathrm{K}\mathrm{kg}\mathrm{mo}{\mathrm{l}}^{-1}$, $0.10\mathrm{mol}\mathrm{k}{\mathrm{g}}^{-1}$ glucose gives about $0.186\mathrm{K}$ depression, while ideally dissociated sodium chloride at the same molality gives about $0.372\mathrm{K}$. Interactions make real concentrated solutions depart from this ideal counting rule.^[ref]

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.