\(\begin{array}{l}
a)\\
2Al + 6{H_2}S{O_4} \to A{l_2}{(S{O_4})_3} + 3S{O_2} + 6{H_2}O\\
Mg + 2{H_2}S{O_4} \to MgS{O_4} + S{O_2} + 2{H_2}O\\
nS{O_2} = \dfrac{{14,56}}{{22,4}} = 0,65\,mol\\
hh:Al(a\,mol),Mg(b\,mol)\\
27a + 24b = 12,9\\
1,5a + b = 0,65\\
 \Rightarrow a = 0,3;b = 0,2\\
\% mAl = \dfrac{{0,3 \times 27}}{{12,9}} \times 100\%  = 62,79\% \\
\% mMg = 100 - 62,79 = 37,21\% \\
b)\\
nA{l_2}{(S{O_4})_3} = \dfrac{{0,3}}{2} = 0,15\,mol\\
mA{l_2}{(S{O_4})_3} = 0,15 \times 342 = 51,3g\\
nMgS{O_4} = nMg = 0,2\,mol\\
mMgS{O_4} = 0,2 \times 120 = 24g\\
c)\\
mhh = 18,525 - 6,45 = 12,075g\\
hh:C{l_2}(a\,mol),{O_2}(b\,mol)\\
71a + 32b = 12,075\\
2a + 4b = 0,15 \times 3 + 0,1 \times 2\\
 \Rightarrow a = 0,125;b = 0,1\\
V = (0,125 + 0,1) \times 22,4 = 5,04l
\end{array}\)