Question

For $Zn + 2HCl \to ZnC{l_2} + {H_2}$,How many moles of hydrogen are produced from the reaction of three moles of zinc with an excess of hydrochloric acid?

Answer 1

During the reaction, whenever two or more than two reactants are required then the reactant whose concentration is in limited quantity will get used up first and due to which the reaction gets stopped and it is known as Limiting reagent. Limiting reagent is the reagent which gets finished during the course of reaction and due to which reaction gets stopped.

Complete step by step answer:
Those chemical reactions which require more than one reactant for the formation of product, then for such reactions the formation of product depends upon the concentration of both reactants. To form products continuously both reactants require according to the stoichiometry of the reaction.
For example: - ${H_2} + C{l_2} \to 2HCl$
For the formation of $HCl$ , we require both the reactant i.e., hydrogen $({H_2})$ and chlorine $(C{l_2})$ in equal quantities. According to the stoichiometry of the reaction we require one mole of hydrogen and one mole of chlorine to form $2$ mole of hydrogen chloride. If any one of the reactants is less than one mole then reaction gets stopped because that reactant gets completely used up during the reaction, as a result reaction gets stopped and the other reactant remains in excess with respect to the first one who is completely used up. The reactant which gets completely used during the reaction is called LIMITING REAGENT while the one who is in excess is called EXCESS REAGENT.
Given equation – $Zn + 2HCl \to ZnC{l_2} + {H_2}$
Here Hydrogen chloride is in excess while we have three moles of zinc.
So according to the given condition zinc is in limiting reagent while hydrogen chloride is excess reagent, so the formation of hydrogen ${H_2}$ depends upon the amount of zinc given. According to the stoichiometry of the reaction
one mole of zinc produces one mole of hydrogen gas
so, three moles of zinc produce three moles of hydrogen gas.
Final answer:
The final answer is $3$ moles of hydrogen gas is produced by $3$ moles of zinc.

Note:
Before finalizing the answer, one should have the balanced chemical reaction because without balancing the reaction, proper calculation of the answer is not possible as we follow the law of conservation of mass during the reaction.