Reaction 9: reaction of zinc and hydrochloric acid
write a complete balanced equation for the reaction

Assuming that zinc displaces the hydrogen as the cation in HCl, and assumes its natural charge of 2+, it requires two Cl- to make the zinc chloride compound. Therefore, we know there need to be two HCl atoms in the equation, and that the resulting compound ZnCl2 requires us to account for two lost hydrogen atoms, which will come out as natural diatomic gas:
Zn(s) + 2HCl(aq) --> ZnCl2(aq) + H2(g)