Mole fraction
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.[2] 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 lower case Greek letter χ (chi) instead of a Roman
. For mixtures of gases, the letter
is recommended.
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[edit] Properties
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.
[edit] Related quantities
[edit] Mass fraction
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.
[edit] Mole percentage
Multiplying mole fraction by 100 gives the mole percentage, also referred as amount/amount percent (abbreviated as n/n%).
[edit] Mass concentration
The conversion to and from mass concentration (chemistry)
is given by:
where
is the average molar mass of the mixture.
[edit] Molar concentration
The conversion to molar concentration
is given by:
where
is the average molar mass of the solution and
is the density of the solution.
[edit] References
- ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "mole fraction".
- ^ http://goldbook.iupac.org/A00296.html
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