The stoichiometric equation of the reaction between magnesium and hydrogen chloride can be written as follows,

One mole magnesium can liberate one mole of hydrogen gas according to the following equation.

 $\underset{\underset{=0.1 \mathrm{mole}}{2.4 \mathrm{g}}}{\mathrm{Mg}}+2\mathrm{HCl}\to {\mathrm{MgCl}}_2+\underset{0.1 \mathrm{mole}}{{\mathrm{H}}_2}$
Volume of $1$ mole ${\mathrm{H}}_2$ at $=22.4 \mathrm{L}$ 
$\therefore 0.1$ mole ${\mathrm{H}}_2$ at STP will occupy $=2.24 \mathrm{L}$