$\text{Đáp án+Giải thích các bước giải:}$

$\text{Câu 21:}$

$\text{2Na + $2H_{2}O$→2NaOH + $H_{2}$}$

$\text{CaO + $H_{2}O$→$Ca(OH)_{2}$}$

$\text{$SO_{3}$ + $H_{2}O$→$H_{2}SO_{4}$}$

$\text{Mg + 2HCl→$MgCl_{2}$ + $H_{2}$}$

$\text{$Fe_{3}O_{4}$ + $4H_{2}$$\xrightarrow{t^o}$3Fe + $4H_{2}$}$

$\text{Câu 22:}$

$\text{ $SO_{3}$ + $H_{2}O$→$H_{2}SO_{4}$}$

$\text{ $H_{2}SO_{4}$ + Mg→$MgSO_{4}$ + $H_{2}$}$

$\text{ CuO + $H_{2}$$\xrightarrow{t^o}$Cu + $H_{2}O$}$

$\text{ 2Cu + $O_{2}$$\xrightarrow{t^o}$2CuO}$

$\text{ 2Ca + $O_{2}$$\xrightarrow{t^o}$2CaO}$

$\text{ CaO + $H_{2}O$→$Ca(OH)_{2}$}$

$\text{ Ca + $2H_{2}O$→$Ca(OH)_{2}$ + $H_{2}$}$

Na+H2O-->NaOH+1/2H2

CaO+H2O-->Ca(OH)2

SO3+H2O-->H2SO4

Mg+2HCl-->MgCl2+H2

4H2+Fe3O4-->3Fe+4H2O

Câu 22:

SO3+H2O-->H2SO4

H2SO4+Mg-->MgSO4+H2

H2+CuO-->Cu+H2O

2Cu+O2-->2CuO

2Ca+O2-->2CaO

CaO+H2O-->Ca(OH)2

Ca+H2SO4-->CaSO4+H2