The vapour density of $\mathrm{N}_2 \mathrm{O}_4$ in $\mathrm{N}_2 \mathrm{O}_4 \rightleftharpoons 2 \mathrm{NO}_2$ is 40 . The degree of dissociation is

What is the equilibrium constant $\left(K_C\right)$ for the given reaction?

$$ \mathrm{N}_2+\mathrm{O}_2 \rightleftharpoons 2 \mathrm{NO} $$

Where the equilibrium concentration of $\mathrm{N}_2, \mathrm{O}_2$ and NO are found to be $4 \times 10^{-3}, 3 \times 10^{-3}$ and $3 \times 10^{-3} \mathrm{M}$ respectively.