(b)
$Cl_{2} + 2l^{-} \rightarrow I_{2} + 2Cl^{-}$
Rate of formation of
$I_{2} = \frac{dI_{2}}{dt} = - \frac{1}{2}\frac{dI^{-}}{dt}$
$= \frac{1}{2} \times \frac{0.20 - 0.18}{20}$
$= \frac{1}{2} \times \frac{0.02}{20}$
$= 5 \times 10^{- 4}mol\ L^{- 1}\min^{- 1}$