Examine the following chemical equation, which shows only the reactants and one product.

Zn + 2HCl -> ZnCl2 + ?

How many H atoms must be present in the second product that is formed?

2 atoms

Explanation:

Given equation:

               Zn   +   2HCl  →  ZnCl₂   +     ?

This is a single displacement reaction. In such reaction, the reactant goes into product when one specie displacement reaction.

Here on specie displaces another in the reaction. The replacement of a metallic ion in solution by a metal atom higher in the activity series than the metal in the solution.

Since Zinc is higher than hydrogen in the acid, it will displace it and produce hydrogen gas.

The complete equation is:

             Zn   +   2HCl  →   ZnCl₂  +  H₂

1 mole of hydrogen gas  is made up of 2 ATOMS OF HYDROGEN

the answer is symmetrical : )