Introduction
Stoichiometry is the part of chemistry that deals with the amounts of substances in a chemical reaction. It helps you figure out how much of each reactant you need and how much product you will get. Think of it like a recipe — if you know how much of one ingredient you have, you can figure out how much of everything else you need.
This stoichiometry calculator makes solving these problems quick and easy. Just enter your balanced chemical equation and the amount of a known substance, and the calculator will do the math for you. It uses mole ratios from the balanced equation to find the amounts of reactants or products. Whether you are working on homework or studying for a test, this tool saves time and helps you check your work.
How to Use Our Stoichiometry Calculator
Enter a chemical equation and a known amount of one substance, and this calculator will find the amount of any other substance in the reaction. It also balances equations, finds limiting reagents, and shows step-by-step solutions.
Chemical Equation: Type your chemical equation into the equation field using standard notation. You can use =, ->, or → to separate reactants from products. For example, type 2 H2 + O2 = 2 H2O or CH4 + 2 O2 -> CO2 + 2 H2O. Click "Balance & Parse" to balance the equation and display molar masses and mole ratios.
Known Substance: Pick the substance you already know the amount of from the "Species" dropdown. Then enter the amount you have in the "Amount" field. Choose whether your amount is in moles, grams, liters at STP, or particles by selecting the matching unit. If you choose grams, the molar mass will fill in automatically, but you can change it if needed. If you need to determine the molecular weight of a compound separately, our dedicated tool can help.
Target Substance: Select the substance you want to solve for from the "Species" dropdown. Then choose the unit you want the answer in — moles, grams, liters at STP, or particles.
Percent Yield: Use the slider or type a number between 1 and 100 to set the percent yield. Leave it at 100% for a theoretical yield. Lower it if you want to account for a real-world reaction that does not fully convert all reactants to products. You can use our percent error calculator to compare your experimental yield against the theoretical value.
Calculate: Click the "Calculate" button to get your result. The calculator will show the final answer, a full step-by-step solution, a table with moles, grams, liters, and particles for every substance in the equation, and a bar chart comparing moles across all species.
Limiting Reagent Analysis: To find the limiting reagent, scroll to the "Limiting Reagent Analysis" section. Enter known amounts for at least two reactants, choose a unit for all inputs, and click "Find Limiting Reagent." The calculator will tell you which reactant runs out first, how much excess remains, and how much of each product can be formed.
What Is Stoichiometry?
Stoichiometry is the branch of chemistry that deals with the amounts of substances involved in chemical reactions. It uses the relationships between reactants and products in a balanced chemical equation to figure out how much of each substance is needed or produced. Think of it like a recipe: if you know how much of one ingredient you have, stoichiometry helps you calculate how much of everything else you need and how much you'll end up with.
How Stoichiometry Works
Every chemical reaction follows the law of conservation of mass, which means atoms are never created or destroyed — they just rearrange. A balanced chemical equation shows this by making sure the number of each type of atom is the same on both sides. The numbers in front of each chemical formula are called coefficients, and they tell you the ratio of moles of each substance. This ratio is called the mole ratio, and it is the foundation of all stoichiometric calculations.
For example, in the reaction 2 H₂ + O₂ → 2 H₂O, the mole ratio tells us that 2 moles of hydrogen gas react with 1 mole of oxygen gas to produce 2 moles of water. If you start with 4 moles of H₂, you can use the mole ratio to determine that you need 2 moles of O₂ and will produce 4 moles of H₂O.
Key Concepts in Stoichiometry
Moles and Molar Mass
The mole is the standard unit chemists use to count particles like atoms and molecules. One mole equals approximately 6.022 × 10²³ particles, a number known as Avogadro's number. The molar mass of a substance is the mass of one mole of that substance, measured in grams per mole (g/mol). You calculate it by adding up the atomic masses of all the atoms in a chemical formula using a molecular weight calculator. For instance, water (H₂O) has a molar mass of about 18.015 g/mol — that's (2 × 1.008) + 15.999.
Converting Between Units
Stoichiometry problems often require converting between moles, grams, liters of gas, and number of particles. Here are the key conversion relationships:
- Grams to moles: Divide the mass by the molar mass.
- Moles to grams: Multiply the moles by the molar mass.
- Moles to liters (gases at STP): Multiply by 22.414 L/mol. STP stands for standard temperature and pressure (0°C and 1 atm). For gas calculations involving temperature and pressure changes, our ideal gas law calculator is a helpful companion tool.
- Moles to particles: Multiply by Avogadro's number (6.022 × 10²³). When working with very large or very small numbers like these, a scientific notation calculator can be useful.
Limiting Reagent
In real-world reactions, you rarely have the exact perfect amounts of every reactant. The limiting reagent is the reactant that runs out first and stops the reaction from continuing. The other reactants are called excess reagents because some of them are left over when the reaction is done. To find the limiting reagent, convert each reactant's amount to moles, then divide by its coefficient in the balanced equation. The reactant with the smallest value is the limiting reagent.
Percent Yield
In a perfect world, a reaction would produce exactly the amount predicted by stoichiometry. That predicted amount is called the theoretical yield. In practice, reactions often produce less due to side reactions, incomplete reactions, or losses during the process. The percent yield tells you how efficient the reaction was:
Percent Yield = (Actual Yield ÷ Theoretical Yield) × 100
Our percentage calculator can help with this and other percentage-based calculations.
Steps to Solve a Stoichiometry Problem
- Write and balance the chemical equation. Make sure the number of atoms of each element is equal on both sides.
- Convert your known quantity to moles. If you're given grams, divide by the molar mass. If given liters of gas at STP, divide by 22.414.
- Use the mole ratio from the balanced equation to find the moles of the target substance.
- Convert from moles to your desired unit — grams, liters, or particles.
- Apply percent yield if the problem asks for the actual amount produced.
Pay attention to significant figures throughout these steps to make sure your final answer reflects the precision of your measurements.
Why Stoichiometry Matters
Stoichiometry is used everywhere in science and industry. Pharmacists use it to make medicines with the right doses. Engineers use it to design rockets and fuel systems. Environmental scientists use it to study pollution and combustion. In chemistry labs, stoichiometry works hand in hand with concepts like molarity to prepare solutions and with dilution calculations to adjust concentrations. Understanding pH in acid-base reactions also depends on accurate stoichiometric reasoning. Whether you are figuring out how much baking soda reacts with vinegar in a science project or calculating industrial chemical production, stoichiometry gives you the math to get accurate answers.