$\therefore \;1 \;mole$ of $\mathrm{HCl}(\mathrm{aq})$ produces $=\frac{3}{6}$ moles of $\mathrm{H}_{2}(\mathrm{g})=\frac{1}{2}\; moles$ of $\mathrm{H}_{2}(\mathrm{g})$
From avogadro hypothesis we know at $STP \;1\; mole$ of any as occupies $22.4 \mathrm{\,L}$
$\therefore \frac{1}{2}$ moles of $\mathrm{H}_{2}(\mathrm{g})$ occupies $=\frac{1}{2} \times 22.4=11.2 \mathrm{\,L}$