P Block Elements

Aqueous solution of a salt \((A)\) forms a dense white precipitate with \(\mathrm{BaCl}_2\) solution. The precipitate dissolves in dilute \(\mathrm{HCl}\) to produce a gas \((B)\) which decolourises acidified \(\mathrm{KMnO}_4\) solution.
\(A\) and \(B\) respectively are