1.14.46: Molality and Mole Fraction

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Page ID: 389971

Michael J Blandamer & Joao Carlos R Reis
University of Leicester & Faculdade de Ciencias

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Molality

For a solution prepared using \(\mathrm{n}_{j}\) moles of solute and \(\mathrm{w}_{\mathrm{s}}\) kg of solvent , molality

\[m_{j}=n_{j} / w_{s}\]

Molality \(\mathrm{m}_{j}\) expressed in ‘\(\mathrm{mol kg}^{-1}\)’ is independent of temperature and pressure being defined by the masses of solvent and solute. The solvent may comprise a mixture of liquids, the composition of the solvent being described using mole fractions, weight-per-cent or volume-per-cent.

Mole Fraction

For a closed system comprising \(\mathrm{n}_{1}, \mathrm{~n}_{2}, \mathrm{~n}_{3} \ldots \mathrm{~n}_{i}\) moles of each \(\mathrm{k}\) chemical substance, the mole fraction of chemical substance \(\mathrm{j}\),

\[\mathrm{x}_{\mathrm{j}}=\mathrm{n}_{\mathrm{j}} / \sum_{\mathrm{k}=1}^{\mathrm{k}=\mathrm{i}} \mathrm{n}_{\mathrm{k}}\]

where

\[\sum_{k=1}^{k=1} x_{k}=1\]

Mole fraction xj is independent of temperature and pressure (in the absence of chemical reaction between the chemical substances in the system).