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Concept
Fraction molaire
Résumé
In chemistry, the mole fraction or molar fraction (xi or χi) is defined as unit of the amount of a constituent (expressed in moles), ni, divided by the total amount of all constituents in a mixture (also expressed in moles), ntot. This expression is given below:
The sum of all the mole fractions is equal to 1:
The same concept expressed with a denominator of 100 is the mole percent, molar percentage or molar proportion (mol%).
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 Ni divided by the total number of all molecules Ntot. The mole fraction is sometimes denoted by the lowercase Greek letter χ (chi) instead of a Roman x. For mixtures of gases, IUPAC recommends the letter y.
The National Institute of Standards and Technology of the United States prefers the term amount-of-substance fraction over mole fraction because it does not contain the name of the unit mole.
Whereas mole fraction is a ratio of moles to moles, molar concentration is a quotient of moles to volume.
The mole fraction is one way of expressing the composition of a mixture with a dimensionless quantity; mass fraction (percentage by weight, wt%) and volume fraction (percentage by volume, vol%) are others.
Mole fraction is used very frequently in the construction of phase diagrams. It has a number of advantages:
it is not temperature dependent (as is 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.
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