An open manometer attached to a flask containing ammonia gas have no difference in mercury level initially as shown in diagram. After sparking into the flask, ammonia is partially dissociated as $2{\text{NH}}_3\left(\text{g}\right)⟶{\text{N}}_2\left(\text{g}\right)+3{\text{H}}_2\left(\text{g}\right)$. Now it have difference of $\text{18}\text{cm}$ in mercury level in two columns, what is partial pressure of ${\text{H}}_{\text{2}}\left(\text{g}\right)$ at equilibrium ?