Em tham khảo nha:
\(\begin{array}{l}
19)\\
MgO + 2HCl \to MgC{l_2} + {H_2}O\\
Fe + 2HCl \to FeC{l_2} + {H_2}\\
{n_{HCl}} = 0,1 \times 2 + 0,1 \times 2 = 0,4\,mol\\
{V_{{\rm{ddHCl}}}} = \dfrac{{0,4}}{1} = 0,4l = 400\,ml\\
20)\\
A{l_2}{O_3} + 6HCl \to 2AlC{l_3} + 3{H_2}O\\
{n_{A{l_2}{O_3}}} = \dfrac{{20,4}}{{102}} = 0,2\,mol\\
{n_{HCl}} = 6{n_{A{l_2}{O_3}}} = 1,2\,mol\\
{C_M}HCl = \dfrac{{1,2}}{{0,3}} = 4M \Rightarrow a = 4
\end{array}\)