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In chemistry, the mole fraction is defined as the amount of a constituent divided by the total amount of all constituents in a mixture :^{[1]}
The sum of all the mole fractions is equal to 1:
The mole fraction is also called the amount fraction.^{[1]} It is identical to the number fraction, which is defined as the number of molecules of a constituent divided by the total number of all molecules . It is one way of expressing the composition of a mixture with a dimensionless quantity (mass fraction is another). The mole fraction is sometimes denoted by the lowercase Greek letter (chi) instead of a Roman .^{[2]}^{[3]} For mixtures of gases, IUPAC recommends the letter .^{[1]}
Mole fraction is used very frequently in the construction of phase diagrams. It has a number of advantages:
The mass fraction can be calculated using the formula
where is the molar mass of the component and is the average molar mass of the mixture.
Replacing the expression of the molar mass:
Multiplying mole fraction by 100 gives the mole percentage, also referred as amount/amount percent (abbreviated as n/n%).
The conversion to and from mass concentration is given by:
where is the average molar mass of the mixture.
The conversion to molar concentration is given by:
or
where is the average molar mass of the solution, c total molar concentration and is the density of the solution .
The mole fraction can be calculated from the masses and molar masses of the components:
In a spatially nonuniform mixture, the mole fraction gradient triggers the phenomenon of diffusion.
