Đáp án:

 1) x=0,35

2) Magie

Giải thích các bước giải:

 \(\begin{array}{l}
1)\\
BTKL:{m_{Fe}} + {m_{{O_2}}} = {m_{hh}}\\
 \Rightarrow {m_{{O_2}}} = {m_{hh}} - {m_{Fe}} = 39,2 - 28 = 11,2g\\
{n_{{O_2}}} = \dfrac{{11,2}}{{32}} = 0,35\,mol \Rightarrow x = 0,35\\
2)\\
BTKL:{m_M} + {m_{hh}} = {m_{cr}}\\
 \Rightarrow {m_{hh}} = {m_{cr}} - {m_M} = 34,5 - 10,8 = 23,7g\\
{n_{hh}} = \dfrac{{8,4}}{{22,4}} = 0,375\,mol\\
hh:C{l_2}(a\,mol),{O_2}(b\,mol)\\
\left\{ \begin{array}{l}
71a + 32b = 23,7\\
a + b = 0,375
\end{array} \right.\\
 \Rightarrow a = 0,3;b = 0,075\\
BT\,e:2{n_M} = 2{n_{C{l_2}}} + 4{n_{{O_2}}}\\
 \Rightarrow {n_M} = \dfrac{{2 \times 0,3 + 4 \times 0,075}}{2} = 0,45\,mol\\
{M_M} = \dfrac{{10,8}}{{0,45}} = 24g/mol \Rightarrow M:Magie(Mg)
\end{array}\)