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):[1] It is denoted xi (lowercase Roman letter x), sometimes χi (lowercase Greek letter chi).
[6] This nomenclature is part of the International System of Quantities (ISQ), as standardized in ISO 80000-9,[4] which deprecates "mole fraction" based on the unacceptability of mixing information with units when expressing the values of quantities.
It has a number of advantages: Differential quotients can be formed at constant ratios like those above: or The ratios X, Y, and Z of mole fractions can be written for ternary and multicomponent systems: These can be used for solving PDEs like: or This equality can be rearranged to have differential quotient of mole amounts or fractions on one side.
or Mole amounts can be eliminated by forming ratios: Thus the ratio of chemical potentials becomes: Similarly the ratio for the multicomponents system becomes The mass fraction wi can be calculated using the formula where Mi is the molar mass of the component i and M̄ is the average molar mass of the mixture.
This property has consequences for representations of phase diagrams using, for instance, ternary plots.
The mole fraction can be calculated from the masses mi and molar masses Mi of the components: In a spatially non-uniform mixture, the mole fraction gradient triggers the phenomenon of diffusion.