6 g of Mg reacts with the excess of an acid. The amount of hydrogen produced would be

$$\begin{gathered} \mathrm{Mg}+2\mathrm{HCl}\to {\mathrm{MgCl}}_2+{\mathrm{H}}_2 \\ 24 \mathrm{g} \mathrm{of} \mathrm{Mg} \mathrm{gives} 2 \mathrm{g} \mathrm{of} {\mathrm{H}}_2 \\ 1 \mathrm{g} \mathrm{of} \mathrm{Mg} \mathrm{gives} \frac{2}{24} \mathrm{g} \mathrm{of} {\mathrm{H}}_2 \\ 6 \mathrm{g} \mathrm{of} \mathrm{Mg} \mathrm{gives} \frac{2}{24} \times 6 \mathrm{g} \mathrm{of} {\mathrm{H}}_2 \end{gathered}$$

The amount of hydrogen produced=$\frac{12}{24}=0.5 \mathrm{g}$