While solving the problems relating to chemical equation, the reactant reacts according to the balanced chemical equation. Quite often, one of the reactant is present in lesser amount while the other may be present in higher amount. The reactant which is present in lesser amount is known as limiting reactant or reagent.

[i] According to the above given reaction, 1 atom of A reacts with 1 molecule of B. Thus, 200 molecules of B will react with 200 atoms of A, thereby leaving 100 atoms of A unused. Hence, B is the limiting reagent as atom B is in lesser amount [200].

[ii] According to the reaction given in the question, 1 mol of A reacts with 1 mol of B. Thus, 2 mol of A will react with only 2 mol of B. As a result, 1 mol of A is not being consumed in the reaction process. Hence, A is the limiting reagent.

[iii] According to the given reaction given in the question, 1 atom of A combines with 1 molecule of B. Thus, all 100 atoms of A will combine with all 100 molecules of B. Hence, the mixture is stoichiometric where no limiting reagent is present.

[iv] 1 mol of atom A combines with 1 mol of molecule B. Thus, 2.5 mol of B will combine with only 2.5 mol of A. As a result, 2.5 mol of A will be left as such as it is not consumed in the reaction. Hence, B is the limiting reagent because B is less as compared to A.

[v] According to the reaction in the question, 1 mol of atom A combines with 1 mol of molecule B. Thus, 2.5 mol of A will combine with only 2.5 mol of B and the remaining 2.5 mol of B will be left as such. Hence, A is the limiting reagent as it is present in lesser amount.