Equilibrium constant for the reaction $H_2O(g) + CO (g) \leftrightharpoons H_2(g) + CO_2 (g)$ is 81. If the velocity constant of the forward reaction is $162\; L\; mol^{-1} s^{-1}$. What is the velocity constant (in $L\;mol^{-1} s^{-1}$) for the backward reaction ?
