We are asked what is the limiting reactant and how many molecules of product can form.

Step 1. Balance the reaction.

A_{2} + B_{2} → AB_{3}

Balance A

A_{2} + B_{2} → 2 AB_{3}

Balance B

**A _{2} + 3 B_{2} → 2 AB**

Step 2. Calculate product base on both reactants.

**A _{2} → 3 molecules**

1 molecule A_{2} : 2 molecules AB_{3}

$\mathbf{3}\mathbf{}\overline{)\mathbf{molecules}\mathbf{}{\mathbf{A}}_{\mathbf{2}}\mathbf{}}\mathbf{\times}\mathbf{}\frac{\mathbf{3}\mathbf{}\mathbf{molecules}\mathbf{}{\mathbf{AB}}_{\mathbf{3}}}{\mathbf{1}\mathbf{}\overline{)\mathbf{molecules}\mathbf{}{\mathbf{A}}_{\mathbf{2}}\mathbf{}}}$

**= 9 molecules AB**_{3}

The scene below represents a mixture of A2 (blue) and B2 (green) before they react to form AB3.

(a) What is the limiting reactant?

(b) How many molecules of product can form?