Answer;
14.0 g of hydrogen gas
Explanation;
The equation for the reaction is
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
From the equation, 2mol HCl will produce 1 mol H2
Moles of HCl =14 moles
Therefore; 14 moles of HCl will produce 7 moles of hydrogen gas
1 mole of Hydrogen gas = 2.0 g/mol
Thus; mass of 7.0 moles of H2 = 7 × 2.0 g/mol
= 14.0 g