The mole fraction of a component in a solution is simply the number of moles of that component divided by the total moles of all the components. The mole fraction of component i is written as Xi. For a solution consisting of nA moles of component A, nB moles of component B, nC moles of component C, etc., then the mole fraction of component A is given by
Mole fractions are strictly additive. The sum of the mole fractions of all components is equal to one.