2mole of $N_2$ is mixed with 6 mol of $H_2$  in a closed vessel of one litre capacity. If 50% of $N_2$ is converted in to $N{H}_3$ at equilibrium, the value of $K_c$  for the reaction $N_2\left(g\right)+3H_2\left(g\right)\underset{}{\overset{}{\rightleftharpoons }}2N{H}_3\left(g\right)$  is