The reaction of magnesium and hydrochloric acid is a single displacement reaction. The reaction is expressed as Mg + 2HCl= MgCl2 + H2. There is a one is to one correspondence of Mg and H2. Hence there is also 0.02 mol of H2. Using PV=nRT where T is 273 K, P is 1 atm and R is 0.0821 L atm/mol K, volume is equal to 0.4483 L or 448.3 mL.