The sum of all the mole fractions is equal to 1:
The mole fraction is also called the amount fraction. 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 . For mixtures of gases, IUPAC recommends the letter .
Mole fraction is used very frequently in the construction of phase diagrams. It has a number of advantages:
- it is not temperature dependent (such as molar concentration) and does not require knowledge of the densities of the phase(s) involved
- a mixture of known mole fraction can be prepared by weighing off the appropriate masses of the constituents
- the measure is symmetric: in the mole fractions x=0.1 and x=0.9, the roles of 'solvent' and 'solute' are reversed.
- In a mixture of ideal gases, the mole fraction can be expressed as the ratio of partial pressure to total pressure of the mixture.
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:
where is the average molar mass of the solution, c total molar concentration and is the density of the solution .
Mass and molar mass
Spatial variation and gradient
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "amount fraction".
- Zumdahl, Steven S. (2008). Chemistry (8th ed. ed.). Cengage Learning. p. 201. ISBN 0-547-12532-1.
- Rickard, James N. Spencer, George M. Bodner, Lyman H. (2010). Chemistry : structure and dynamics. (5th ed. ed.). Hoboken, N.J.: Wiley. p. 357. ISBN 978-0-470-58711-9.