a)
$CaO + H_2O \to Ca(OH)_2$
$CO_2 + H_2O \rightleftharpoons H_2CO_3$
$N_2O_5 + H_2O \to 2HNO_3$
b)
$CaO + 2HCl \to CaCl_2 + H_2O$
$Fe_2O_3 + 6HCl \to 2FeCl_3 + 3H_2O$
$MgO + 2HCl \to MgCl_2 + H_2O$
c)
$CO_2 + KOH \to KHCO_3$
$N_2O_5 + 2KOH \to 2KNO_3 + H_2O$