If 40.0 g of hcl react with an excess of magnesium metal, what is the theoretical yield of hydrogen?

The reaction between HCl and Mg is a substitution reaction:
2HCl + Mg --> MgCl2 + H2

The molar mass of HCl is 36.46 g/mol. Converting the amount of HCl,
40 g / 36.46 g/mol = 1.0971 mol

Based on the balanced chemical reaction, the stiochiometric ratio is:
2 mol HCl / 1 mol H2

So,
1.0971 mol HCl (1 mol H2 / 2 mol HCl) = 0.549 mol H2

Converting moles to mass
0.549 mol H2 (2.02 g H2 / mol H2) = 1.108 g H2

The theoretical yield is:
(1.108/40) (100) = 2.77

The yield is 2.77 g H2 per 100 g of HCl