Question

how many liters of hydrogen gas at STP are produced from the reaction of 50.0g of magnesium and 75g of hydrochloric acid?

Answer 1

Mg + 2HCl ==> MgCl2 + H2
mols Mg = g/atomic mass = 50/24.3 = 2.06
mols HCl = 75/36.45 = 2.06
NOTE: Normally when I see a quantity given for more than one of the reactants it turns out that the problem is a limiting reagent (LR) problem. In this case, however, moles of each is the same so you need to work with one reactant only.
2.06 moles Mg will product 2.06 moles H2 since 1 mol Mg produces 1 mol H2 gas.
1 mol H2 gas occupies 22.4 L @ STP so 2.06 x 22.4 L/mol = ? L H2 gas.