Define mole fraction.

(N/A) Mole fraction is defined as the ratio of the number of moles of a particular component to the total number of moles of all components present in the solution.
If a substance $A$ dissolves in substance $B$ and their number of moles are $n_{A}$ and $n_{B}$ respectively,then the mole fractions of $A$ and $B$ are given as:
Mole fraction of $A$ $(x_{A})$ = $\frac{n_{A}}{n_{A} + n_{B}}$
Mole fraction of $B$ $(x_{B})$ = $\frac{n_{B}}{n_{A} + n_{B}}$
The sum of mole fractions of all components in a solution is always equal to $1$ (i.e.,$x_{A} + x_{B} = 1$).