= Amount fraction
{title2=$x_i=n_i/\sum_jn_j$}
The <amount fraction> of component $i$ is $x_i=n_i/\sum_jn_j$, where $n_i$ is its <amount of substance>. It is dimensionless and the fractions sum to one for a complete composition specification.
For an <ideal gas mixture> at common <temperature> and <pressure>, <amount fraction> equals partial-pressure fraction. It generally differs from <mass fraction> because different species have different <molar masses>. For one mole each of hydrogen and oxygen <molecules>, both <amount fractions> are one half, although their <masses> are very different. The composition basis must therefore accompany a quoted concentration.
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