Em tham khảo nha :

\(\begin{array}{l}
4)\\
a)\\
2Mg + {O_2} \to 2MgO\\
b)\\
{n_{Mg}} = \dfrac{m}{M} = \dfrac{{7,2}}{{24}} = 0,3mol\\
{n_{MgO}} = {n_{Mg}} = 0,3mol\\
{m_{MgO}} = n \times M = 0,3 \times 40 = 12g\\
c)\\
{n_{{H_2}}} = \dfrac{{{n_{Mg}}}}{2} = 0,15mol\\
{V_{{H_2}}} = n \times 22,4 = 0,15 \times 22,4 = 3,36l\\
5)\\
Zn + 2HCl \to ZnC{l_2} + {H_2}\\
{n_{ZnC{l_2}}} = \dfrac{m}{M} = \dfrac{{68}}{{136}} = 0,5mol\\
{n_{Zn}} = {n_{ZnC{l_2}}} = 0,5mol\\
{m_{Zn}} = n \times M = 0,5 \times 65 = 32,5g\\
6)\\
CTHH:A{l_x}{(S{O_4})_3}\\
M = 342dvC\\
 \Rightarrow x{M_{Al}} + 3 \times ({M_S} + 4{M_O}) = 342\\
 \Rightarrow x = \dfrac{{342 - 3 \times (32 + 4 \times 16)}}{{27}} = 2\\
 \Rightarrow CTHH:A{l_2}{(S{O_4})_3}
\end{array}\)