The balanced reaction is:
Mg (s) + 2HCl (aq) = H2 (g) + MgCl2 (aq)We are given the amount of reactants to be reacted. This will be the starting point of the calculations.
.125 L HCl solution (6.0 mol HCl / L solution) ( 1 mol H2 / 2 mol HCl ) ( 2.02 g H2 / 1 mol H2 ) = 0.76 g H2