$a)$
$S+O_{2} \xrightarrow{t^o} SO_{2}$
$2SO_{2}+O_{2} \xrightarrow{t^o} 2SO_{3}$
$SO_{3}+H_{2}O \to H_{2}SO_{4}$
$3H_{2}SO_{4}+Al_{2}O_{3} \to Al_{2}(SO_{4})_{3}+3H_{2}O$
$b)$
$CaCO_{3} \xrightarrow{t^o} CaO+CO_{2} \uparrow$
$CaO+H_{2}O \to Ca(OH)_{2}$
$Ca(OH)_{2}+2HCl \to CaCl_{2}+2H_{2}O$