$3)$
$(1):\ 2Fe(OH)_{2}+4H_{2}SO_{4\ \text{đặc}} \to Fe_{2}(SO_{4})_{3}+SO_{2} \uparrow+6H_{2}O$
$4)$
Muối $X$ là $Ba(HCO_{3})_{2}$
$Ba(HCO_{3})_{2}+H_{2}SO_{4} \to BaSO_{4} \downarrow+2H_{2}O+2SO_{2} \uparrow$
$Ba(HCO_{3})_{2}+2NaOH \to BaCO_{3} \uparrow+Na_{2}CO_{3}+2H_{2}O$