mole fraction (X)
itz moles of d elemnt divided by total no of moles
likes if there r 4 moles of Oxygen and 6moles of hydrogen (just an xample)
so mole fraction of oxygen is 4/(10)
here 10=total no of moles 
always X(oxygen)+X(hydrogen)=1
sum of mole fractions of all d elemts present=1