The reduction of peroxydisulphate ion by ${\mathrm{I}}^{-}$ ion is expressed by ${\mathrm{S}}_2{\mathrm{O}}_8^{2-}+3{\mathrm{I}}^{-}\to 2{\mathrm{SO}}_4^{2-}+{\mathrm{I}}_3^{-}$. If rate of disappearance of ${\mathrm{I}}^{-}$ is $9/2\times {10}^{-3} \mathrm{mol} {\mathrm{L}}^{-1} {\mathrm{S}}^{-1}$, what is the rate of formation of ${\mathrm{SO}}_4^{2-}$ during same time?