The equilibrium $S{O}_2\left(g\right)+\frac{1}{2}{O}_2\left(g\right)\rightleftharpoons S{O}_3\left(g\right)$  is established in a container of 4L at a particular temperature.  If the number of moles of $S{O}_2,O_2$  and $S{O}_3$  at equilibrium are 2, 1 and 4 respectively. Then find the value of equilibrium constant $(K_c in{L}^{1/2}mo{l}^{-1/2})$.