Stoichiometry Calculator
Calculate limiting reactant, product amounts, theoretical yield, and percent yield from a balanced chemical equation. This page is built for classroom stoichiometry, lab prep checks, and quick yield verification when you already know the reaction setup.
Edited by Gail Joyce
Gail Joyce edits chemistry calculator pages for formula clarity, unit consistency, and realistic classroom and laboratory workflow guidance.
This page is maintained as a stoichiometry workflow reference. The equation handling, yield language, limiting-reactant notes, worked examples, and support sections are reviewed against standard general chemistry references before major updates.
Stoichiometry Calculator
Enter a balanced chemical equation and reactant amounts to calculate product amounts, limiting reactant, and theoretical yield. Supports mole, mass, and particle-count inputs.
Scope: this page is for balanced-equation stoichiometry, limiting-reactant checks, and yield math. It is not a reaction balancer, kinetics solver, or equilibrium calculator.
How to Use the Stoichiometry Calculator
Follow the same order you would use while solving a stoichiometry worksheet: confirm the balanced equation, enter the known reactants, decide whether you want mass or mole input, and then review the limiting-reactant logic in the result.
Enter a balanced chemical equation first
Type the reaction with proper coefficients. This page uses those coefficients as the mole-ratio backbone for every product, limiting-reactant, and yield result.
Choose whether your input is in moles or grams
Use moles if you already converted the starting material, grams when you want mass-based entries, or particles when you are working from molecule or formula-unit counts.
Enter each reactant you actually know
Add the reactant formula and amount for each starting material you want included. The limiting reactant will be identified from the scaled reaction units implied by the balanced equation.
Use the optional fields only if they match your goal
Add a target product if you want the result focused on one output. Add actual yield only if you want the page to also compute percent yield from your experimental result on the same displayed basis.
Table of Contents
Quickly navigate to different sections of this guide. Click any item below to jump to that section.
Understanding Stoichiometry
Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in chemical reactions. It's like a recipe for chemistry—it tells you exactly how much of each ingredient (reactant) you need to make a certain amount of product. The word "stoichiometry" comes from Greek roots meaning "element" and "measure," which perfectly describes what it does: it measures the elements and compounds involved in reactions.
At the heart of stoichiometry are balanced chemical equations, which show the mole ratios between reactants and products. These ratios allow you to predict how much product you'll get from given amounts of reactants, or how much reactant you need to produce a desired amount of product. Our Stoichiometry Calculator makes these calculations instant and accurate, whether you're solving homework problems, planning laboratory experiments, or designing industrial processes.
Key Concepts in Stoichiometry
Limiting Reactant
The limiting reactant (also called limiting reagent) is the reactant that runs out first in a chemical reaction. It determines the maximum amount of product that can be formed. Once the limiting reactant is consumed, the reaction stops, even if other reactants are still present. Identifying the limiting reactant is crucial for predicting reaction yields.
Excess Reactant
The excess reactant is any reactant that remains after the reaction is complete. There's more than enough of this reactant to react with the limiting reactant. Knowing which reactant is in excess helps you understand how much will be left over and can guide you in optimizing reaction conditions.
Theoretical Yield
Theoretical yield is the maximum amount of product that can be formed based on stoichiometric calculations. It assumes perfect conditions, complete reaction, and no side reactions or losses. In practice, actual yields are usually less than theoretical yields due to various factors like incomplete reactions, side reactions, and product losses during purification.
Percent Yield
Percent yield compares the actual yield (what you actually got) to the theoretical yield (what you should have gotten). It's calculated as (Actual Yield / Theoretical Yield) × 100%. High percent yields (close to 100%) indicate efficient reactions, while low percent yields suggest problems like incomplete reactions, side reactions, or product losses.
Common Stoichiometric Relationships
| Reaction | Mole Ratio | Example |
|---|---|---|
| 2H₂ + O₂ → 2H₂O | 2:1:2 | 2 mol H₂ produce 2 mol H₂O |
| N₂ + 3H₂ → 2NH₃ | 1:3:2 | 1 mol N₂ produces 2 mol NH₃ |
| 2Na + Cl₂ → 2NaCl | 2:1:2 | 2 mol Na produce 2 mol NaCl |
| CaCO₃ → CaO + CO₂ | 1:1:1 | 1 mol CaCO₃ produces 1 mol CaO |
| C₃H₈ + 5O₂ → 3CO₂ + 4H₂O | 1:5:3:4 | 1 mol C₃H₈ produces 3 mol CO₂ |
Formulas and Equations
Stoichiometric calculations are based on mole ratios from balanced chemical equations. Our Stoichiometry Calculator does all the math for you, but understanding the formulas helps you see what's happening behind the scenes:
Mole Ratio from Balanced Equation
The coefficients in a balanced equation give you the mole ratios. For example, in 2H₂ + O₂ → 2H₂O, the ratio is 2 mol H₂ : 1 mol O₂ : 2 mol H₂O.
To find product moles from reactant moles, multiply by the ratio of coefficients. From 2 mol H₂, you get 2 mol H₂O (since the ratio is 2:2).
Finding the Limiting Reactant
Compare how much product each reactant can produce:
For each reactant:
Product Moles = Reactant Moles × (Product Coefficient / Reactant Coefficient)
The reactant that produces the least product is the limiting reactant.
Percent Yield Formula
Compare what you actually got (actual yield) to what you should have gotten (theoretical yield). Multiply by 100% to express as a percentage.
Percent yield is always ≤ 100%. If it's greater than 100%, check your calculations—you may have measurement errors or a different product than expected.
Worked Examples
Step-by-step solutions demonstrating how to solve stoichiometry problems. These examples show you how to use the Stoichiometry Calculator effectively.
Example 1: Water Formation
Scenario: In the reaction 2H₂ + O₂ → 2H₂O, you have 4.0 moles of H₂ and 2.0 moles of O₂. How many moles of H₂O can be produced? What is the limiting reactant?
Solution:
From H₂: 4.0 mol H₂ × (2 mol H₂O / 2 mol H₂) = 4.0 mol H₂O
From O₂: 2.0 mol O₂ × (2 mol H₂O / 1 mol O₂) = 4.0 mol H₂O
Both produce the same amount, so both are limiting (stoichiometric mixture).
Answer: Theoretical Yield = 4.0 mol H₂O. Both reactants are limiting (stoichiometric amounts).
Example 2: Limiting Reactant Identification
Scenario: In the reaction N₂ + 3H₂ → 2NH₃, you have 1.0 mol N₂ and 2.0 mol H₂. What is the limiting reactant and theoretical yield?
Solution:
From N₂: 1.0 mol N₂ × (2 mol NH₃ / 1 mol N₂) = 2.0 mol NH₃
From H₂: 2.0 mol H₂ × (2 mol NH₃ / 3 mol H₂) = 1.33 mol NH₃
H₂ produces less product, so H₂ is the limiting reactant.
Answer: Limiting Reactant = H₂, Theoretical Yield = 1.33 mol NH₃
Example 3: Percent Yield Calculation
Scenario: The theoretical yield of a reaction is 25.0 g, but the actual yield is 20.0 g. What is the percent yield?
Solution:
Percent Yield = (Actual Yield / Theoretical Yield) × 100%
Percent Yield = (20.0 g / 25.0 g) × 100% = 80.0%
Answer: Percent Yield = 80.0%
Common Mistakes
Most stoichiometry errors come from setup, not arithmetic. These quick checks catch the mistakes that most often break limiting-reactant and yield results.
Using an unbalanced equation
All mole-ratio logic comes from the balanced coefficients. If the equation is not balanced, every limiting-reactant and product result is wrong before the calculator even starts.
Mixing grams and moles mentally
When you choose mass input, the page must convert through molar mass first. Treating grams as if they were already moles leads to completely misleading comparisons between reactants.
Forgetting that the smallest scaled amount limits the reaction
The limiting reactant is not always the one with the smaller starting mass. It is the reactant that supports the fewest reaction units after dividing by the balanced coefficient.
Comparing actual yield against the wrong product basis
If you enter actual yield, make sure it matches the same product and unit basis as the theoretical yield. Otherwise the percent-yield output becomes meaningless.
Frequently Asked Questions (FAQs)
These are the main questions worth keeping close to the calculator workflow.
What does this stoichiometry calculator solve?
It converts known reactant amounts into reaction-unit support, identifies the limiting reactant, and reports theoretical yield for one target product or all products in the balanced equation.
Why must the equation be balanced first?
All stoichiometric conversions come from the balanced coefficients. If the equation is wrong, the limiting-reactant call, product amounts, and percent-yield math will all be wrong too.
How does the calculator find the limiting reactant?
It converts each entered reactant to moles, divides by the balanced coefficient, and compares the resulting reaction units. The smallest scaled value is the limiting reactant.
Can I work from grams or particles instead of moles?
Yes. This page supports mole, gram, and particle-count inputs. Grams are converted through molar mass, and particles are converted with Avogadro’s number.
What happens if I leave target product blank?
The calculator reports theoretical yields for every product in the balanced equation. Use the target-product field only when you want one focused output.
How should I enter actual yield for percent-yield math?
Enter actual yield on the same basis as the displayed target-product result. If the product result is shown in grams, actual yield should also be grams; the same applies to moles or particles.
Error messages usually indicate unbalanced equations, missing reactant data, invalid formulas, or calculation errors. Check that your equation is balanced, all reactants have formulas and amounts, and formulas use standard notation.
References and Further Reading
For more in-depth information about stoichiometry, chemical reactions, and related topics, consult these authoritative sources:
| Resource | Description | Category |
|---|---|---|
| ChemLibreTexts: Stoichiometric Calculations | Open reference overview of stoichiometric mole-ratio calculations | General Chemistry |
| OpenStax Chemistry 2e: Reaction Yields | Textbook treatment of limiting reactants, theoretical yield, and percent yield | General Chemistry |
| Brown, T. L., LeMay, H. E., Bursten, B. E., et al. Chemistry: The Central Science | General chemistry text covering balanced equations, limiting reactants, and yield math | General Chemistry |
| LibreTexts Chemistry | Open-access chemistry textbooks and resources | General Chemistry |
| PubChem | Database of chemical properties and structures | Chemical Data |
| NIST Chemistry WebBook | Standard reference data for chemical compounds | Chemical Data |
| ChemSpider | Free chemical structure database | Chemical Data |
| Royal Society of Chemistry | Professional chemistry resources and publications | Professional |