b
\(Na_2S\) and \(NaCN\), formed during fusion with metallic sodium, must be removed before adding \(AgNO_3\), otherwise black ppt. due to \(Na_2S\) or white precipitate due to \(AgCN\) will be formed and thus white precipitate of \(AgCl\) will not be identified easily.

  \(N{a_2}S + 2AgN{O_3} \to 2NaN{O_3} + \mathop {A{g_2}S}\limits_{Black}  \downarrow \)

\(NaCN + AgN{O_3} \to NaN{O_3} + \mathop {AgCN}\limits_{White}  \downarrow \)

\(NaCl + AgN{O_3} \to NaN{O_3} + \mathop {AgCl}\limits_{white}  \downarrow \)

\(N{a_2}S + 2HN{O_3}\xrightarrow{{boil}}2NaN{O_3} + {H_2}S \uparrow \)

\(NaCN + HN{O_3}\xrightarrow{{boil}}NaN{O_3} + HCN \uparrow \)