Chemistry - Complete d-block Question with Solution | TestHub

To remove OH- ion a weak acid$\left({\mathrm{H}}_2\mathrm{C}{\mathrm{O}}_3\right)$is required because a strong acid can reverse the reaction. So here HCl is a strong acid so it reverse the reaction.

KOH is strong base which will increase OH- concentration.

$\mathrm{S}{\mathrm{O}}_2$will react with water to form a strong acid${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$.

$\mathrm{C}{\mathrm{O}}_2$combines with$\mathrm{O}{\mathrm{H}}^{-}$ion to give carbonate which is easily removed.