Mole fraction of species $i$ $(x_i)$ $x_i=\frac{no\ of\ moles\ of\ species\ i}{no\ of\ moles\ of\ the\ mixture} = \frac{C_i}{C}$ $\sum_{i=1}^N x_i =1$ Relation to [[Mass Fraction]] $x_i = \frac{y_i/w_i}{\sum_{i=1}^N y_i/wi}$