Question

In a reaction \(A + B_2 → AB_2\) Identify the limiting reagent, if any, in the following reaction mixtures.

1. 300 atoms of A + 200 molecules of B
2. 2 mol A + 3 mol B
3. 100 atoms of A + 100 molecules of B
4. 5 mol A + 2.5 mol B
5. 2.5 mol A + 5 mol B

Answer 1

Reaction and Stoichiometry

Balanced reaction: A + B2 -> AB2 Stoichiometric ratio: \[ n(\text{A}) : n(\text{B}_2) = 1 : 1 \] So, compare the amounts of A and B₂ directly.

Case (i): 300 atoms A + 200 molecules B₂

Required: 300 A needs 300 B₂ (1:1), but only 200 B₂ are present. Therefore, B₂ is limiting and A is in excess.

Case (ii): 2 mol A + 3 mol B₂

Required: 2 mol A needs 2 mol B₂. Present: 3 mol B₂. Therefore, A is limiting and B₂ is in excess.

Case (iii): 100 atoms A + 100 molecules B₂

Required: 100 A needs 100 B₂. Present: 100 A and 100 B₂. Neither is in excess; no limiting reagent, both are completely consumed.

Case (iv): 5 mol A + 2.5 mol B₂

Required: 5 mol A needs 5 mol B₂, but only 2.5 mol B₂ are present. Therefore, B₂ is limiting and A is in excess.

Case (v): 2.5 mol A + 5 mol B₂

Required: 2.5 mol A needs 2.5 mol B₂. Present: 5 mol B₂. Therefore, A is limiting and B₂ is in excess.

Summary Table

Case	Given Amounts	Limiting Reagent
(i)	300 atoms A, 200 molecules B₂	B₂
(ii)	2 mol A, 3 mol B₂	A
(iii)	100 atoms A, 100 molecules B₂	None (both just sufficient)
(iv)	5 mol A, 2.5 mol B₂	B₂
(v)	2.5 mol A, 5 mol B₂	A