\(\begin{array}{l}
a)\\
Dat\,hoa\,tri\,A\,la\,n(n\in\mathbb{N^*})\\
2A+nCl_2\xrightarrow{t^o}2ACl_n\\
Theo\,PT:\,n_A=n_{ACl_n}\\
\to \frac{5,4}{A}=\frac{26,7}{A+35,5n}\\
\to A=9n\to \begin{cases} n=3\\ A=27 \end{cases}\\
b)\\
n_{Al}=\frac{5,4}{27}=0,2(mol)\\
2Al+2NaOH+2H_2O\to 2NaAlO_2+3H_2\\
Theo\,PT:\,n_{H_2}=1,5n_{Al}=0,3(mol)\\
\to V_{H_2(đktc)}=0,3.22,4=6,72(l)\\
Theo\,PT:\,n_{NaOH}=n_{Al}=0,2(mol)\\
\to m_{dd\,NaOH}=\frac{0,2.40}{8\%}=100(g)\\
\to V_{dd\,NaOH}=\frac{100}{1,115}\approx 89,7(ml)
\end{array}\)