if 4.5 g of Zn reacts with 5.0 g of HCl which is the limiting reactant

Answer 1

Zn + 2HCl --> ZnCl2 + H2
mols Zn = grams/atomic mass = 4.5 /65.4 = approx 0.07
mols HCl = 5.0/36.5 = approx 0.14

Now change mols Zn and HCl to mols ZnCl2 or H2 formed. You do this by using the coefficients in the balanced equation.

0.07 mols Zn x (1 mol ZnCl2/1 mol Zn) = approx 0.07 mols ZnCl2.
0.14 mols HCl x (1 mol ZnCl2/2 mols Hcl) = approx 0.07 mols ZnCl2.

These numbers are so close that you must redo them more accurately. The one producing the smaller number of mols ZnCl2 will be the limiting reagent.