How many moles of hydrogen are produced from the reaction of three moles of zinc with an excess of hydrochloric acid

Okay, start with a chemical equation:

Zn + HCl - - > ZnCl2 + H2:

The oxidation state of Zn is 2+, while Cl is 1-, so you need 2 Cls for every Zn. Also, Hydrogen is a diatomic element, so it exists in H2, not H.

Next, balance it

Zn + 2HCl - - > ZnCl2 + H2

Next, notice that for every mole of Zn in the product, there is exactly 1 mole of H2 : a 1:1 mole ratio.

So, if 3 moles of Zn go in, 3 moles of H2 come out.