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Magnesium reacts with excess dilute hydrochloric acid in the apparatus shown: $\mathrm{Mg+2HCl\rightarrow MgCl_2+H_2}$. At the end, the gas syringe contains $96\,\mathrm{cm^3}$ of hydrogen. One mole of gas occupies $24\,\mathrm{dm^3}$ under the conditions used, and $A_r(\mathrm{Mg})$ $=24$. What mass of magnesium reacted?