Equilibrium constant can also be expressed in terms of Kx, when concentration of the species are taken in mole fractionF2(g)⇌  2F(g);  Kx = XF2XF2For the above equilibrium mixture, average molar mass at 1000 K was 36.74 g c Thus, Kx is

Let, F2(g)=x (mole fraction),molar mass = 38.09 mol-1F(g) = (1 - x), molar mass = 19.0 g  mol-1∴   Average molar mass = M1x1 + M2x2x1 + x236.74 = 38 x + 19 (1 - x)∴    x = 0.9337 (mole faction of F2)(1 - x) = 0.0633 (mole fraction of F)∴   Kx = XF2XF2=0.066320.9337 = 4.708× 10-3