What is the theoretical yield of hydrogen gas if 5.00 mol of zinc are added to an excess of hydrochloric acid?

So in this problem, your limiting reactant is the 5.00 mol of Zn, because you have an excess of hydrochloric acid.

The first step to answering this problem is to set up a balanced equation.

You start with Zn + HCl ⇒ ZnCl + H

In order to balance it, you have to know that Zinc is a + 2 ion, Cl is a - 1 ion, and H is a + 1 ion. Each molecule must be balanced (meaning ionic charges equal to zero), as well as the number of each element on either side of the reaction.

This gives you Zn + 2HCl ⇒ ZnCl₂ + 2H

Now, in order to find how much Hydrogen gas you have, you have to know how many atoms of Zn you have.

1 mol is 6.022 * 10²⁴ atoms (Avogadro's Number), so if you have 5 mol of Zn, you have 3.011 * 10²⁵ atoms of zinc.

Plug this into the reaction and re-balance the formula.

(3.011 * 10²⁵ Zn) + (6.022 * 10²⁵ HCl) ⇒ (3.011 * 10²⁵ ZnCl₂) +

(6.022 * 10²⁵ H)

You end up with (6.022 * 10²⁵ H), or 10.00 mol H (g)