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The mole fraction is a measure of the concentration of a component substance in a mixture of substances. It is defined as the number of moles of a component substance in a mixture divided by the total number of moles of the mixture. It may be expressed as:

$x_{\mathrm {a} }={\frac {n_{\mathrm {a} }}{n_{\mathrm {a} }+n_{\mathrm {b} }+n_{\mathrm {c} }\ldots }}={\frac {n_{\mathrm {a} }}{n_{\mathrm {total} }}}$

where:
$x_{\mathrm {a} }$	= the mole fraction of component $\mathrm {a}$ in the mixture
$n_{\mathrm {a} }$	= the number of moles of component $\mathrm {a}$ in the mixture
$n_{\mathrm {b} }$	= the number of moles of component $\mathrm {b}$ in the mixture
$n_{\mathrm {c} }$	= the number of moles of component $\mathrm {c}$ in the mixture
$n_{\mathrm {total} }$	= the total number of moles in the mixture

Mole fractions are dimensional numbers and they are often referred to as mol fractions.

Applications

Mole fractions are very commonly used to express concentrations of specific substances in gas mixtures and liquid solutions in chemistry, thermodynamics, chemical engineering and other disciplines.