Amount fraction ($x_i=n_i/{\sum }_j{n}_j$)

The amount fraction of component $i$ is $x_i=n_i/{\sum }_j{n}_j$, 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.