In chemistry, the mole fraction or molar fraction (xi) is defined as 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: In chemistry, the mole fraction or molar fraction (xi) is defined as 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: