Given the reaction below, and your knowledge of the "Law of Conservation of Mass", what is the missing product from the chemical reaction?

How do you know?

Zn(s) + HCl(aq) --> ___ + ZnCl2(aq)

Single Replacement Oxidation Reduction Rxn

Zn(s) + 2HCl(aq) => ZnCl2(aq) + H2(g)