In first case reactions involved are$$\begin{gathered} \mathrm{Reduction} \mathrm{of} {\mathrm{Fe}}^{+++}, {\mathrm{Fe}}^{+++}\to {\mathrm{Fe}}^{++} \\ \mathrm{Titration} \mathrm{step}, 6{\mathrm{Fe}}^{++}+{\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\to 6{\mathrm{Fe}}^{+++}+2{\mathrm{Cr}}^{+++} \\ \mathrm{By} \mathrm{using} \mathrm{titration} \mathrm{reaction}, \mathrm{solve} \mathrm{the} \mathrm{question} \end{gathered}$$

 In second case reaction involved is${\mathrm{As}}_4{\mathrm{O}}_6+4{{\mathrm{I}}_3}^{-}\to {\mathrm{As}}_4{\mathrm{O}}_{10}+12{\mathrm{I}}^{-}$Now solve

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