Question

In the reaction$Zn + 2HCl \to ZnC{l_2} + {H_2}$ , how many moles of hydrogen will be formed when $4$ moles of $HCl$ are consumed?

Answer 1

This is an example of redox reaction. Here the zinc is oxidized and the hydrogen is reduced. The reaction occurs between one mole of zinc and two moles of hydrochloric acid.

Complete step by step answer:
When a chemical reaction occurs definite moles of one reactant react with definite moles of reagents to generate products. However the reagents may be present in excess. A chemical reaction is represented with the help of chemical equations.
The chemical equation is then balanced by following the law of conservation of mass. The number of elements in the reactant and product side is the same. Thus from the balanced chemical equation we get the number of moles of reactant taking part in the reaction as well as the number of moles of products formed after the reaction.
The mole is a unit of measurement used for indicating the amount of chemicals or compounds in chemistry. Thus the given reaction is the reaction of metallic zinc with hydrochloric acid which produces a salt zinc chloride and hydrogen gas. The corresponding equation is written as
$Zn(s) + 2HCl(aq) \to ZnC{l_2}(aq) + {H_2}(g)$
The above equation is a balanced chemical equation as the number of atoms of each element is the same on both sides. From the equation we say that $1$ mole of zinc reacts with $2$ moles of $HCl$ to give $1$ mole of $ZnC{l_2}$ and $1$ mole of hydrogen. The mole ratios are $1:2:1:1$.
In this case we need to determine the moles of hydrogen using the moles of $HCl$. The mole ratio of $HCl$ and hydrogen is $2:1$. Thus $2$ moles of hydrochloric acid produce $1$ mole of hydrogen gas.
Hence when $4$ moles of $HCl$ are consumed produces $2$ moles of hydrogen as the concentration of reactants and products is doubled.

Note: The mole concept is very useful in chemistry calculations. The reactions are put on by calculating the exact amount of reactants using the number of moles of all the reacting species.