You divide the moles of each substance by its coefficient of the balanced equation. The substance that has the smallest answer is the limiting reactant.
Consider the reaction:
2Al + 3I₂ → 2AlI₃
What is the limiting reactant if 1.20 g Al reacts with 2.40 g I₂?
Step 1: Convert all masses into moles.
1.20 g Al ×
2.40 g I₂ ×
Step 2: Divide the moles of each substance by its coefficient in the balanced equation.
For Al: 0.0445/2 = 0.0222
For I₂: 0.00946/3 = 0.003 15
Since I₂ gives the smaller number, I₂ is the limiting reactant.
Here is a video which discusses how to determine limiting reactants.
video from: Noel Pauller