Ishwar S. answered • 05/08/19
University Professor - General and Organic Chemistry
Zn + 2 HCl → ZnCl2 + H2
Since you are given moles of Zn and HCl, convert the moles of one reactant to the other. Then compare the calculated # of moles to the one used in the reaction (i.e., given in the question).
If the calculated # of moles is LESS than the given amount, then that reactant is the limiting reagent, however, if the calculated # of moles is MORE than the given amount, then that reactant is in excess.
For instance, let's convert mol Zn to mol HCl. From the balanced chemical reaction, you can see that for every 1 mol of Zn that reacts, 2 mol of HCl are needed. This ratio is called the mole ratio in stoichiometry.
4.3 mol Zn x (2 mol HCl / 1 mol) = 8.6 mol HCl --> This tells us that 8.6 mol HCl is needed to react completely with 4.3 mol of Zn. Since only 7.6 mol of HCl is used, HCl is the limiting reagent, and Zn is in excess.
To confirm, let's convert 7.6 mol HCl to mol Zn.
7.6 mol HCl x (1 mol Zn / 2 mol HCl) = 3.8 mol Zn --> This tell's us that only 3.8 mol of Zn is needed to completely react with 7.6 mol of HCl. However, since 4.3 mol of Zn is used for the reaction, Zn is therefore, the excess reactant.
FYI - mol of Zn in excess = 4.3 - 3.8 = 0.5 mol
Hope the above was helpful.
