The reduction of peroxysulphate ion by $I^-$ ion is expressed by $S_2O_8^{2-} + 3I^- \to 2SO_4^{2-} + I^{3-}$. If rate of disappearance of $I^-$ is $\frac{9}{2} \times 10^{-3}$ mol lit$^{-1}$ s$^{-1}$, what is the rate of formation of $SO_4^{2-}$ during same time?