The mole fraction of a compound is another measure of concentration. Mole fraction is simply the number of moles of a particular substance compared to the total number of moles. Since the number of moles of one substance in a mixture can never be more than the total number of moles, the mole fraction for a substance always ranges between 0 (none) and 1 (all). The mole fraction is typically given the symbol X. The mole fraction for a substance \(i\) is defined as
\[X_i = {n_i\over n_{\rm total}}\]
where \(n_{\rm total}\) is the total number of moles of all the substances in the mixture. When dealing with partial pressures in gas mixtures it is important to consider only moles of the substances that are in the gas phase as these are the only ones that contribute to the pressure.