Limiting Reactant Solver
Find the limiting reactant, the excess reactant, the leftover mass, and the theoretical yield of every product for any chemical reaction, with full step-by-step working.
Type your equation, enter how much of each reactant you have, and find which one runs out first.
Limiting reactant
H2
Excess reactant: N2
0.99206
Reaction extent (mol)
33.792 g
Total product mass
0.208 g
Excess mass left over
2
Reactants used
Reactant Usage & Product Yield
Compare how many moles you have against how many moles the reaction actually needs.
Step-by-Step Limiting Reactant Calculation
Here's exactly how this answer was calculated, one step at a time.
Given: N2 + H2 -> NH3
Step 1: Balance the chemical equation
Every limiting reactant calculation starts from a balanced equation, since the coefficients set the exact mole ratio the reactants must react in.
N2 + 3 H2 -> 2 NH3Step 2: Find the molar mass of every reactant
M(N2) = 28.014 g/mol | M(H2) = 2.016 g/molStep 3: Convert each given amount into moles
Mass alone can't be compared directly between different substances, so every quantity is converted to moles first using n = m ÷ M.
N2: 28 g = 0.9995 mol | H2: 6 g = 2.97619 molStep 4: Divide each mole amount by its coefficient
Dividing by the coefficient scales every reactant down to the same basis, so the smallest result marks the reactant that runs out first.
N2: 0.9995 ÷ 1 = 0.9995 | H2: 2.97619 ÷ 3 = 0.99206Step 5: Identify the limiting reactant
H2 gives the smallest ratio (0.99206), so H2 is the limiting reactant.Step 6: Calculate how much of each reactant is actually used
N2: 0.99206 mol used | H2: 2.97619 mol usedStep 7: Calculate leftover (excess) reactant
Whatever moles aren't consumed by the reaction remain unreacted once the limiting reactant runs out.
N2: 0.00744 mol left over = 0.2083 g | H2: 0 mol left over = 0 gStep 8: Calculate the theoretical yield of every product
NH3: 1.98413 mol = 33.7917 gStep 9: Check the result with conservation of mass
Since atoms are only rearranged, not created or destroyed, the total mass of reactants consumed should equal the total mass of products formed.
Mass consumed (33.7917 g) = mass of products formed (33.7917 g) ✓
Limiting reactant:
H2
Limiting Reactant Calculator: Find the Limiting Reagent Instantly
This limiting reactant calculator, also called a limiting reagent calculator, tells you which reactant in a chemical reaction runs out first. Just type your equation, enter how much of each reactant you're starting with, and it works out the limiting reactant, the excess reactant, how much of that excess is left over, and the theoretical yield of every product — all in one go.
It's built for chemistry students working through stoichiometry homework, teachers checking answers, and anyone in a lab who needs a fast, reliable way to plan a reaction without doing the mole math by hand. Every answer comes with a full step-by-step solution underneath it, so you can either check your own working or learn the method properly as you go.
What Is a Limiting Reactant?
In most real reactions, you don't mix your reactants in the exact ratio the equation calls for. One of them usually gets used up before the other. That one is called the limiting reactant, or limiting reagent, because it limits how much product the reaction can make. Once it's gone, the reaction simply stops, no matter how much of the other reactant is still sitting there unused.
The reactant that's left over once the reaction stops is called the excess reactant. It isn't wrong to have extra of it — in fact, chemists often add a slight excess of a cheaper reactant on purpose, just to make sure the more expensive or more important reactant reacts completely.
How to Find the Limiting Reactant Step by Step
First, write a balanced chemical equation — the coefficients in front of each formula tell you the exact mole ratio the reactants combine in. Second, convert every amount you're given, usually in grams, into moles, using the molar mass of each substance. You can't compare reactants fairly while they're still in grams, since a gram of one substance and a gram of another almost never contain the same number of moles.
Third, divide each reactant's moles by its coefficient in the balanced equation. This scales every reactant down to the same basis, so the numbers become directly comparable. Whichever reactant gives the smallest result after this division is the limiting reactant — it's the one that runs out first, and it sets the maximum amount of product the reaction can form. This calculator carries out every one of these steps automatically and shows the full working underneath the result.
The Formula This Calculator Uses
For each reactant, the calculator computes moles available using n = mass ÷ molar mass, then divides that figure by the reactant's coefficient in the balanced equation. The smallest of these values across all reactants is called the reaction extent, and it represents how many complete 'sets' of the reaction can actually take place.
From the reaction extent, the calculator multiplies back through every other coefficient to find exactly how much of each reactant gets used, how much is left over as excess, and how much of every product forms. This is the same method taught in every general chemistry course, just carried out with exact arithmetic instead of a calculator and a pen.
What Is Theoretical Yield, and How Is It Connected?
Theoretical yield is the maximum amount of product a reaction can make, assuming everything reacts perfectly with no losses. It's calculated directly from the limiting reactant — you can never make more product than the limiting reactant allows for, no matter how much excess reactant is sitting in the flask.
This calculator reports the theoretical yield of every product in both moles and grams, right alongside the limiting reactant result, since the two numbers are really just two views of the same calculation. If you want to go further and compare a real lab result against this theoretical figure, that's what percent yield measures.
Limiting Reactant vs Excess Reactant vs Theoretical Yield
These three ideas often get mixed up, so it helps to keep them separate. The limiting reactant is the substance that runs out first and stops the reaction. The excess reactant is whatever is left unreacted once the limiting reactant is gone — this calculator tells you exactly how many grams of it remain.
Theoretical yield, on the other hand, isn't a reactant at all — it's the maximum mass of product the reaction can form, calculated entirely from the limiting reactant's amount. Knowing all three together gives you the complete picture of a reaction: what ran out, what's left over, and how much you should expect to get out the other end.
Worked Example: Limiting Reactant in Ammonia Production
Take the industrially important reaction N2 + 3H2 -> 2NH3. Suppose you start with 28 grams of nitrogen gas and 6 grams of hydrogen gas. Nitrogen's molar mass is about 28.0 g/mol, so 28 g gives roughly 1.00 mol of N2. Hydrogen's molar mass is about 2.02 g/mol, so 6 g gives roughly 2.98 mol of H2.
Dividing by the coefficients gives 1.00 ÷ 1 = 1.00 for nitrogen, and 2.98 ÷ 3 ≈ 0.992 for hydrogen. Since 0.992 is smaller, hydrogen is actually the limiting reactant here, even though it looks like there's plenty of it compared to nitrogen — this is exactly the kind of trap that makes limiting reactant problems tricky without a proper mole-ratio calculation, and exactly what this calculator is built to catch.
Common Mistakes Students Make With Limiting Reactant Problems
The single biggest mistake is comparing reactants by mass instead of by moles. Having more grams of one reactant than another tells you almost nothing about which one will run out first, because different substances have very different molar masses. Always convert to moles before comparing anything.
A second common mistake is forgetting to divide by the coefficient before comparing. Two reactants can have the same number of moles available and still not be equally 'limiting,' if the balanced equation needs three times as much of one as the other. A third mistake is using an unbalanced equation — the whole calculation depends on the coefficients being correct, so this calculator always balances the equation first, automatically.
Why the Limiting Reactant Matters in Real Chemistry
Outside the classroom, limiting reactant calculations decide how a real chemical process is run. A production chemist scaling up a reaction needs to know exactly how much of each raw material to order, and how much product a batch will realistically yield, before committing money and time to it.
In large-scale industrial processes, like the Haber process for making ammonia or the production of pharmaceuticals, getting the limiting reactant calculation right affects cost, safety, and how efficiently raw materials are used. Getting it wrong can mean wasted reactant, an incomplete reaction, or a batch that doesn't meet its expected yield.
How to Enter Your Reaction Correctly
Type your equation using standard chemical notation — capital letters to start each element symbol, a lowercase letter if the symbol has two letters, and a number right after an element for its subscript, like Ca(OH)2 or C6H12O6. Separate compounds on the same side with a plus sign, and separate reactants from products using an arrow (->) or an equals sign.
You don't need to balance the equation yourself first — this calculator balances it automatically before running the limiting reactant calculation. Once the equation is entered, a box appears for every reactant so you can type in how much of it you're starting with, in whichever unit is most convenient — milligrams, grams, kilograms, millimoles, or moles.
Limiting Reactant in Everyday Industries
Limiting reactant thinking shows up far beyond a chemistry classroom. Fertiliser manufacturers calculate it to plan exactly how much ammonia and nitric acid a batch of ammonium nitrate needs. Metal refiners use it to work out how much reducing agent is needed to extract a metal from its ore without wasting excess material.
Even everyday baking follows the same logic — if a recipe needs two eggs per cup of flour and you only have one egg, the egg is your limiting reactant, and it caps how much batter you can actually make, no matter how much flour is left in the bag. The underlying math is identical, whether it's a kitchen or an industrial reactor.
Checking Your Own Homework Answer
If you've already worked through a limiting reactant problem by hand, type the same equation and amounts into this calculator and compare your answer to the result. If your limiting reactant matches but your leftover mass or theoretical yield doesn't, look closely at your molar mass calculation — a small error there is the most common source of a mismatched final number.
If your limiting reactant doesn't match at all, check the step where you divided moles by the coefficient. It's easy to divide by the wrong coefficient, especially in equations with several reactants, and this calculator's step-by-step breakdown shows that exact division for every reactant so you can spot where the two answers diverge.
Limiting Reactant Solver FAQ and Quick Reference
To find the limiting reactant, balance the equation, convert every reactant's amount to moles, divide each by its coefficient, and the smallest result identifies the limiting reactant. Everything left over once that reactant runs out is the excess reactant, and the maximum product it can form is the theoretical yield.
This free online limiting reactant calculator is built for homework help, exam revision, and quick reaction planning. For regulated, safety-critical, or large-scale industrial use, always double-check every formula and quantity against a certified reference before relying on it for a real procedure.
Frequently Asked Questions
How do you find the limiting reactant?
Balance the equation, convert every reactant's amount to moles, then divide each reactant's moles by its coefficient in the balanced equation. The reactant with the smallest result is the limiting reactant.
Why can't I just compare reactants by grams?
Different substances have different molar masses, so the same number of grams can represent very different numbers of moles. Comparing moles per coefficient, not grams, is what actually identifies the limiting reactant.
What is the difference between a limiting and an excess reactant?
The limiting reactant runs out first and stops the reaction. The excess reactant is whatever amount of the other reactant is left over, unreacted, once the limiting reactant is used up.
How is theoretical yield related to the limiting reactant?
Theoretical yield is the maximum amount of product a reaction can form, and it's calculated entirely from the limiting reactant's amount — the excess reactant has no effect on it.
Does this calculator balance the equation for me?
Yes. Type your equation in any unbalanced form, and it's balanced automatically before the limiting reactant calculation runs.
Can I enter my reactant amount in moles instead of grams?
Yes. Each reactant has its own unit selector, so you can enter milligrams, grams, kilograms, millimoles, or moles, whichever matches the data you have.