Mole fraction is one way to describe the composition of a mixture: the amount of one substance, in moles, as a proportion of the total moles present. Treating the total as 1, each component's mole fraction is simply its share.

Formula

If component i has n_i moles and all components together have n moles, its mole fraction is x_i = n_i / n. The mole fractions of all components always add up to 1, and the quantity has no unit.

Example

A mixture contains 2 mol of component A and 3 mol of component B, 5 mol in total. Then x_A = 2/5 = 0.4 and x_B = 3/5 = 0.6, which add up to 1.

Points to note

• Mole fraction is based on amount of substance, not mass. To start from masses, first convert each mass to moles using the component's molar mass.
• In an ideal gas mixture, the partial pressure of a component equals its mole fraction multiplied by the total pressure.