Step 1
The reaction provided here:
Zn + 2 HCl → ZnCl2 + H2 (it must be completed and balanced)
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Step 2
Data provided:
3 moles of Zn
5 moles of HCl
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Step 3
Procedure:
(The limiting reactant and the excess reactant refer to as reactants. The limiting reactant limits the formation of the product and it is the one that is consumed completely. The excess reactant is the one that is left behind in excess)
By stoichiometry,
Zn + 2 HCl → ZnCl2 + H2 (the mole rate between Zn and HCl = 1:2)
1 mole Zn ----------- 2 moles HCl
3 moles Zn ----------- X
X = 3 moles Zn x 2 moles HCl/1 mole Zn
X = 6 moles HCl
Now, for 3 moles of Zn, 6 moles of HCl are needed, but there are 5 moles of HCl, so the limiting reactant is HCl, and the excess is Zn.
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The other way round:
1 mole Zn ----------- 2 moles HCl
X ----------- 5 moles HCl
X = 2.5 moles Zn (2.5 moles of Zn are needed, but there are 3 moles, it is the excess)
Answer: b. HCl