(I) Salt ${ }^{\text{'}}{\mathrm{A}}^{\mathrm{\prime }}+\text{ dil. }{\mathrm{H}}_2{\mathrm{SO}}_4{\to }^{\mathrm{\prime }}{\mathrm{B}}^{\mathrm{\prime }}$  gas [ 'B' has suffocating odour]

(ii) Salt$\text{ ' }A\text{ ' }+\mathrm{Pb}(\mathrm{OAc}{)}_2\to C(\downarrow \left)\stackrel{\mathrm{\Delta }}{⟶\text{'}}{D}^{\text{'}}\right(\downarrow )$  [ 'C' and 'D' both are white ppt.]

Here, anion of salt 'A' and 'D' are :

The reaction between dilute sulfuric acid and sodium sulphite is a twofold replacement reaction that takes place in two phases.

Sulphuric acid interacts with sodium sulphite in the first stage to produce sodium sulphate and sulphurous acid.

${\mathrm{H}}_2{\mathrm{SO}}_4+{\mathrm{Na}}_2{\mathrm{SO}}_3\to {\mathrm{Na}}_2{\mathrm{SO}}_4+{\mathrm{H}}_2{\mathrm{SO}}_3$

Highly unstable sulphuric acid spontaneously breaks down into sulphur dioxide and water.

$\begin{array}{l}{\mathrm{H}}_2{\mathrm{SO}}_3\to {\mathrm{SO}}_2+{\mathrm{H}}_2\mathrm{O}\\ {\mathrm{H}}_2{\mathrm{SO}}_4+{\mathrm{Na}}_2{\mathrm{SO}}_3\to {\mathrm{Na}}_2{\mathrm{SO}}_4+{\mathrm{SO}}_2+{\mathrm{H}}_2\mathrm{O}\end{array}$

Sulphur di oxide gas is obtained as the result,

${\mathrm{H}}_2{\mathrm{SO}}_4+{\mathrm{Na}}_2{\mathrm{SO}}_3\to {\mathrm{Na}}_2{\mathrm{SO}}_4+{\mathrm{H}}_2{\mathrm{SO}}_3\to {\mathrm{SO}}_2+{\mathrm{H}}_2\mathrm{O}$

Hence, anion of salt A is ${\mathrm{SO}}_3^{2-}$.