$\left\{ \begin{align}
& X:a(mol);{{M}_{X}} \\
& Y:\,2a(mol);\frac{5}{3}{{M}_{x}} \\
& Z:4a(mol);\frac{7}{3}{{M}_{x}} \\
\end{align} \right.$
a) Khi tác dụng với HCl, các kim loại đều có hóa trị II
$\begin{align}
& {{n}_{KL}}={{n}_{{{H}_{2}}}}\Rightarrow a+2a+4a=\frac{3,136}{22,4}=0,14 \\
& \Rightarrow a=0,02 \\
\end{align}$
$\begin{align}
& {{m}_{KL}}=6,56={{M}_{X}}\times a+\frac{5}{3}{{M}_{X}}\times 2a+\frac{7}{3}{{M}_{X}}\times 4a \\
& a=0,02\Rightarrow {{M}_{X}}=24(Mg) \\
& {{M}_{Y}}=\frac{5}{3}{{M}_{X}}=40(Ca) \\
& {{M}_{Z}}=\frac{7}{3}{{M}_{X}}=56(Fe) \\
\end{align}$
b)
$A\left\{ \begin{align}
& MgC{{l}_{2}}:0,02mol \\
& CaC{{l}_{2}}:0,04mol \\
& FeC{{l}_{2}}:0,08mol \\
\end{align} \right.$
Khi cho A tác dụng với $C{{l}_{2}}$ xảy ra phản ứng:
$\begin{align}
& 2FeC{{l}_{2}}+C{{l}_{2}}\to 2FeC{{l}_{3}} \\
& 0,08\,\,\,\,\,\,\,\,\,\,\,\,0,04\,\,\,\,\,\,\,\,\,\,0,08 \\
\end{align}$
${{m}_{muoi}}={{m}_{MgC{{l}_{2}}}}+{{m}_{CaC{{l}_{2}}}}+{{m}_{FeC{{l}_{3}}}}=0,02\times 95+0,04\times 111+0,08\times 162,5=19,34g$