Question

How many moles of `Zn` will be needed to completely react with 0.4 moles of `HCl` in the reaction `Zn + HCl -> ZnCl_2 + H_2`?

Answer 1

`Zn(s) + 2HCl(aq) rarr ZnCl_2(aq) + H_2(g)`.

`0.2` moles of zinc metal are required for reaction with 0.4 moles of hydrochloric acid.

Explanation:

The equation above is stoichiometrically balanced. What does this mean? It means that for every reactant particle, there is a corresponding product particle. Stoichiometry is also practised in commerce: for every debit item (each charge made to an account), there must be a corresponding credit item (a deposit to made to another account). This principle of "rob Peter, and pay Paul" is fundamental to stoichiometry.

You started with `0.4` `mol` of `HCl(aq)`; I could have started with 4 atoms, or 40 atoms, or 40 moles. By the stoichiometry of the reaction, each zinc particle requires 2 hydrochloric acid particles (i.e. each equivalent of zinc metal requires 2 hydrochloric acid particles). Thus `0.2` mol of zinc metal were required.

Capisce?