Limiting Reactant Calculator

The Limiting Reactant Calculator takes any balanced chemical equation and the amounts of each reactant, then finds the limiting reactant (the one that runs out first), the excess reactant left over, and the theoretical yield of every product. Molar masses are computed automatically from the formulas, and a visual reaction-extent chart shows exactly which reactant is the bottleneck. You can also enter an actual yield to get the percent yield.

What Is a Limiting Reactant?

In most real reactions the reactants are not mixed in perfect stoichiometric proportions. The limiting reactant (also called the limiting reagent) is the reactant that is completely used up first. Once it is gone, the reaction stops — no more product can form, no matter how much of the other reactants remain. The reactants that are left over are called excess reactants. Think of making sandwiches: with 10 slices of bread and 3 slices of cheese, cheese is the limiting "reactant" — you can only make 3 sandwiches and 4 slices of bread are left over.

How to Find the Limiting Reactant

The reliable method is the mole ratio (reaction extent) method:

1. Balance the equation so the coefficients give the correct mole ratios.
2. Convert each reactant to moles by dividing its mass by its molar mass (skip this if you already have moles).
3. Divide moles by the coefficient for each reactant. This value is the reaction extent — how many "batches" of the reaction that reactant can support.
4. The smallest value is the limiting reactant. Everything else is in excess.

Theoretical Yield Formula

Once you know the reaction extent \( \xi \) of the limiting reactant, the amount of each product is:

Excess Reactant Left Over

For each reactant that is not limiting, the amount consumed is its coefficient times the reaction extent. Whatever remains is the excess:

Percent Yield

The theoretical yield is the best case. The actual yield measured in the lab is usually lower because of side reactions, losses, and incomplete reactions. Percent yield compares the two:

Worked Example

Consider the formation of water: 2 H₂ + O₂ → 2 H₂O, starting with 4 g of H₂ and 32 g of O₂.

H₂ has the smaller value (0.99), so hydrogen is the limiting reactant. The reaction extent is 0.99 mol, producing 2 × 0.99 = 1.98 mol of H₂O (about 35.6 g). A small amount of O₂ is left over as the excess reactant.

Common Mistakes

• Forgetting to balance the equation. Coefficients are essential — comparing raw moles without dividing by coefficients gives the wrong answer.
• Comparing grams instead of moles. Always convert to moles first; equal masses are not equal numbers of particles.
• Dividing by the wrong number. Divide moles by the reactant's own coefficient, not the product's.
• Assuming the smallest mass is limiting. The reactant present in the smallest mass is not always the limiting one — molar mass and coefficients matter.

How to Use This Calculator

1. Enter a balanced equation such as 2 H2 + O2 -> 2 H2O. Use -> or = for the arrow and + between species. Amount boxes appear automatically for each reactant.
2. Enter each reactant's amount and pick grams or moles.
3. (Optional) Enter an actual yield and choose a product to get the percent yield.
4. Click Calculate to see the limiting reactant, the reaction-extent chart, the products formed, the excess left over, and a full step-by-step solution.

Frequently Asked Questions

What is a limiting reactant?

The limiting reactant (or limiting reagent) is the reactant that is completely consumed first in a chemical reaction. It limits how much product can form, because once it runs out the reaction stops, even if other reactants remain.

How do you find the limiting reactant?

Convert each reactant amount to moles, then divide the moles of each reactant by its coefficient in the balanced equation. The reactant with the smallest result is the limiting reactant, because it can support the fewest cycles of the reaction.

What is the excess reactant?

The excess reactant is any reactant that is not fully used up. After the limiting reactant runs out, the leftover amount of each other reactant is the excess. It equals the starting moles minus the moles consumed by the reaction.

What is theoretical yield?

Theoretical yield is the maximum amount of product that can be made from the limiting reactant, assuming the reaction goes to completion with no losses. It is found by multiplying the reaction extent by the product coefficient and then by the product molar mass.

Does the equation need to be balanced?

Yes. The coefficients in a balanced equation give the mole ratios used to find the limiting reactant and the yields. If your equation is not balanced, balance it first, otherwise the mole ratios and results will be incorrect.

How is percent yield calculated?

Percent yield equals the actual yield divided by the theoretical yield, multiplied by 100. The actual yield is what you measured in the lab, and the theoretical yield is the maximum predicted by stoichiometry.

Additional Resources

• Limiting Reagent - Wikipedia
• Yield (Chemistry) - Wikipedia