\(\begin{array}{l}
a)\\
4Fe{S_2} + 11{O_2} \to 2F{e_2}{O_3} + 8S{O_2}\\
2S{O_2} + {O_2} \to 2S{O_3}\\
S{O_3} + {H_2}O \to {H_2}S{O_4}\\
CuO + {H_2}S{O_4} \to CuS{O_4} + {H_2}O\\
CuS{O_4} + BaC{l_2} \to CuC{l_2} + BaS{O_4}\\
b)\\
Mn{O_2} + 4HCl \to MnC{l_2} + C{l_2} + 2{H_2}O\\
2Fe + 3C{l_2} \to 2FeC{l_3}\\
FeC{l_3} + 3NaOH \to Fe{(OH)_3} + 3NaCl\\
2Fe{(OH)_3} + 3{H_2}S{O_4} \to F{e_2}{(S{O_4})_3} + 6{H_2}O\\
F{e_2}{(S{O_4})_3} + 3BaC{l_2} \to 2FeC{l_3} + 3BaS{O_4}
\end{array}\)