Question

5.2 grams of magnesium metal are place in 400.0 mL of 1.0 M hydrochloric acid. The balanced equation for the reaction is:
Mg + 2HCl = MgCl2 + H2. How many moles of H2 gas were produced?

Answer 1

mols Mg = g/atomic mass = 5.2/24.3 = about 0.21 but you need a more accurate figure.
mols HCl = L x M = 0.4 x 1M = about 0.4

Convert mols Mg to mols H2 = about 2 x (1 mol H2/1 mol Mg) = about 0.21 mol H2
Convert mols HCl to mols H2. That's 0.4 x (1 mol H2/2 mols HCl) = about 0.2 mol H2
You see the values don't agree for mols H2 produced. This is a limiting reagent problem and the correct value is ALWAYS the smaller value; therefore, 0.2 mol will be H2 produced and HCl is the limiting reagent.