Mole fraction

Proportion of a constituent in a mixture

In chemistry, the mole fraction or molar fraction, also called mole proportion or molar proportion, is a quantity defined as the ratio between the amount of a constituent substance, ni (expressed in unit of moles, symbol mol), and the total amount of all constituents in a mixture, ntot (also expressed in moles): x i = n i n t o t {\displaystyle x_{i}={\frac {n_{i}}{n_{\mathrm {tot} }}}} It is denoted xi (lowercase Roman letter x), sometimes χi (lowercase Greek letter chi). (For mixtures of gases, the letter y is recommended.) It is a dimensionless quantity with dimension of N / N {\displaystyle {\mathsf {N}}/{\mathsf {N}}} and dimensionless unit of moles per mole (mol/mol or mol⋅mol−1) or simply 1; metric prefixes may also be used (e.g., nmol/mol for 10−9). When expressed in percent, it is known as the mole percent or molar percentage (unit symbol %, sometimes "mol%", equivalent to cmol/mol for 10−2).

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