\(a,\) Đặt \(\left\{{}\begin{matrix}n_{Al}=x\left(mol\right)\\n_{Mg}=y\left(mol\right)\end{matrix}\right.\)
\(n_{H_2}=\dfrac{7,84}{22,4}=0,35\left(g\right)\\ PTHH:2Al+6HCl\rightarrow2AlCl_3+3H_2\\ Mg+2HCl\rightarrow MgCl_2+H_2\)
Theo đề ta có HPT: \(\left\{{}\begin{matrix}27x+24y=7,5\\1,5x+y=0,35\end{matrix}\right.\Rightarrow\left\{{}\begin{matrix}x=0,1\left(mol\right)\\y=0,2\left(mol\right)\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}m_{Al}=0,1\cdot27=2,7\left(g\right)\\m_{Mg}=7,5-2,7=4,8\left(g\right)\end{matrix}\right.\)
\(b,n_{HCl}=3n_{Al}+2n_{Mg}=0,3+0,4=0,7\left(mol\right)\\ \Rightarrow m_{CT_{HCl}}=0,7\cdot36,5=25,55\left(g\right)\\ \Rightarrow m_{dd_{HCl}}=\dfrac{25,55\cdot100\%}{14,6\%}=175\left(g\right)\)