In chemistry, the

**mole fraction** or

**molar fraction** (

**x**_{i}) is defined as the amount of a constituent (expressed in moles),

*n*_{i}, divided by the total amount of all constituents in a mixture (also expressed in moles),

*n*_{tot}:

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In chemistry, the **mole fraction** or **molar fraction** (**x**_{i}) is defined as the amount of a constituent (expressed in moles), *n*_{i}, divided by the total amount of all constituents in a mixture (also expressed in moles), *n*_{tot}:

The sum of all the mole fractions is equal to 1:

The same concept expressed with a denominator of 100 is the **mole percent** or **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 *N*_{i} divided by the total number of all molecules *N*_{tot}. 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.

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