Em tham khảo nha :
\(\begin{array}{l}
4)\\
A\\
Ba{(OH)_2} + N{a_2}C{O_3} \to BaC{O_3} + 2NaOH\\
5)\\
C\\
6)\\
B\\
7)\\
D\\
8)\\
B\\
2X + 2{H_2}O \to 2XOH + {H_2}\\
{n_{{H_2}}} = \dfrac{{2,24}}{{22,4}} = 0,1mol\\
{n_X} = 2{n_{{H_2}}} = 0,2mol\\
{M_X} = \dfrac{{7,8}}{{0,2}} = 39dvC\\
X:Kali(K)
\end{array}\)