For the reaction : ${\mathrm{MnO}}_2+4\mathrm{HCl}\stackrel{100\mathrm{\%}}{⟶}{\mathrm{MnCl}}_2+{\mathrm{Cl}}_2+2{\mathrm{H}}_2\mathrm{O}$ 8.7 gm  ${\mathrm{MnO}}_2$ is dissolved in 500 ml of HCI solution containing 7.3 gm HCI per litre. (Mn= 55)

since the Mg will be in excess not all of it will react HCl is the limiting reagent.

$\begin{array}{r}{\underset{0.1 \mathrm{m}}{{\mathrm{MnO}}_2}}_{ }+4\mathrm{HCl}⟶{\mathrm{MnCl}}_2+\underset{0.1 \mathrm{m}}{{\mathrm{Cl}}_2}+2{\mathrm{H}}_2\mathrm{O}\\ \underset{{\mathrm{n}}_{{\mathrm{MnCl}}_2}= 0.025 }{{\mathrm{n}}_{{\mathrm{Cl}}_2}=0.025}{\mathrm{V}}_{{\mathrm{Cl}}_2}=0.025\times 22.4=560\mathrm{ml}\\ \end{array}$