How do you find the limiting reagent?

In a reaction, the reactants are rarely mixed in exactly the proportions of the balanced equation. The one that is present in the smallest relative amount limits how much product can form.

What are the steps?

1. Moles = mass ÷ molar mass, for each reactant.
2. Moles per coefficient = moles ÷ the reactant's coefficient in the balanced equation.
3. The limiting reagent is the reactant with the smallest moles per coefficient. That value is the extent of the reaction.
4. Theoretical yield (mol) of a product = extent × its coefficient. Multiply by its molar mass for grams.
5. Percent yield = actual yield ÷ theoretical yield × 100.

What does a worked example look like?

Aspirin forms from salicylic acid and acetic anhydride in a 1:1 ratio. A student uses 2.00 g of salicylic acid (C7H6O3, 138.12 g/mol) and 5.40 g of acetic anhydride (C4H6O3, 102.09 g/mol).

• Salicylic acid: 2.00 ÷ 138.12 = 0.01448 mol.
• Acetic anhydride: 5.40 ÷ 102.09 = 0.05290 mol.
• With a 1:1 ratio, the smaller value is limiting, so salicylic acid is the limiting reagent.
• Theoretical aspirin (C9H8O4, 180.16 g/mol): 0.01448 × 180.16 = 2.61 g.
• If the student collects 2.10 g, percent yield = 2.10 ÷ 2.61 × 100 = 80.5%.
• Acetic anhydride left over: (0.05290 − 0.01448) × 102.09 = 3.92 g.

What are the common mistakes?

• Comparing masses or raw moles instead of moles per coefficient. With a coefficient of 2, a reactant needs twice as many moles as one with a coefficient of 1.
• Using an equation that is not balanced.
• Treating the mass weighed as pure reagent. Correct for purity and moisture before calculating.
• Forgetting that yields are theoretical maxima at full conversion, so side reactions and losses on transfer cut the real result.

The molar masses above come from the IUPAC standard atomic weights. The limiting reagent and yield calculator finds the limiting reagent with INDEX and MATCH, and also sizes the charge for a scale-up. The same mass-to-moles step appears in how to calculate molarity.