Limiting Reagent Calculator

What Is a Limiting Reagent?

In any chemical reaction, the limiting reagent (also called the limiting reactant) is the substance that is completely consumed first — it determines how much product can be formed and brings the reaction to a stop. The other reactant is called the excess reagent — some of it is left over after the reaction ends.

The Sandwich Analogy

Imagine making sandwiches: each sandwich needs 2 slices of bread and 3 slices of meat. If you have 10 slices of bread and 18 slices of meat:

• Bread allows: 10/2 = 5 sandwiches
• Meat allows: 18/3 = 6 sandwiches
• Bread is the limiting reagent — you can only make 5 sandwiches.
• Meat left over: 18 - (5 × 3) = 3 slices excess.

Chemical reactions work identically — but instead of sandwiches, we use the stoichiometric coefficients from the balanced equation.

The Method

Step 1: Convert each reactant mass to moles: moles = mass / molar mass

Step 2: Divide each moles value by its stoichiometric coefficient: moles_A / coeff_A and moles_B / coeff_B

Step 3: The smaller result identifies the limiting reagent.

Step 4: Calculate theoretical yield: moles_limiting × (coeff_product / coeff_limiting) × molar_mass_product

Step 5: If actual yield is given: % yield = (actual yield / theoretical yield) × 100

Real-World Example — The Haber Process

N₂ + 3H₂ → 2NH₃ (making ammonia, used in fertilizers)

If you have 28 g of N₂ (MM = 28.02 g/mol) and 9 g of H₂ (MM = 2.016 g/mol):

• Moles N₂ = 28/28.02 = 0.999 mol. Ratio: 0.999/1 = 0.999
• Moles H₂ = 9/2.016 = 4.464 mol. Ratio: 4.464/3 = 1.488

N₂ has the smaller ratio → N₂ is the limiting reagent.

Theoretical yield of NH₃ = 0.999 × (2/1) × 17.03 = 34.02 g of ammonia

Why This Matters Industrially

Industrial chemists carefully control reagent ratios to minimize waste and maximize yield. In pharmaceutical synthesis, the most expensive reagent is usually kept as the limiting reagent to ensure it is fully consumed. Percent yield tells chemists how efficient their process is — a 95% yield is excellent; 60% means significant product is lost.

Percent Yield

Theoretical yield is the maximum possible based on stoichiometry. Actual yield is what you collect from the experiment — always less, due to incomplete reactions, side reactions, losses during transfer, and measurement errors.

% yield = (actual yield / theoretical yield) × 100