\(\begin{array}{l}
1)\\
a)\\
MgO + 2HCl \to MgC{l_2} + {H_2}O\\
b)\\
nMgO = \dfrac{4}{{40}} = 0,1\,mol\\
nHCl = 2nMgO = 0,2\,mol\\
\Rightarrow {C_M}HCl = \dfrac{{0,2}}{{0,4}} = 0,5M\\
c)\\
nMgC{l_2} = nMgO = 0,1\,mol\\
\Rightarrow mMgC{l_2} = 0,1 \times 95 = 9,5g\\
2)\\
2KMn{O_4} \xrightarrow{t^0} {K_2}Mn{O_4} + Mn{O_2} + {O_2}\\
C{l_2} + {H_2} \xrightarrow{as} 2HCl\\
F{e_2}{O_3} + 6HCl \to 2FeC{l_3} + 3{H_2}O\\
FeC{l_3} + 3NaOH \to Fe{(OH)_3} + 3NaCl\\
2Fe{(OH)_3} \xrightarrow{t^0}F{e_2}{O_3} + 3{H_2}O\\
3)\\
C{l_2} + S{O_2} + 2{H_2}O \to {H_2}S{O_4} + 2HCl\\
4C{l_2} + {H_2}S + 4{H_2}O \to {H_2}S{O_4} + 8HCl
\end{array}\)
