Stoichiometry Calculator
Calculate moles and mass of reactants and products from a balanced chemical equation using mole ratios.
Stoichiometry: The Math of Chemical Reactions
Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction. It allows you to predict exactly how much of a product will form — or how much reactant you need — for any given amount.
The Mole Ratio
The coefficients in a balanced chemical equation are the mole ratios. They tell you exactly how many moles of each substance react or are produced relative to each other.
Example: Combustion of hydrogen 2 H₂ + O₂ → 2 H₂O
Mole ratios:
- 2 mol H₂ : 1 mol O₂ : 2 mol H₂O
- To produce 4 mol water, you need 4 mol H₂ and 2 mol O₂.
The Three-Step Method
- Convert the given quantity to moles (using molar mass if given in grams).
- Apply the mole ratio from the balanced equation.
- Convert back to the desired unit (moles, grams, or liters for gases).
Step 1: moles = mass ÷ molar mass
Step 2: moles of product = moles of reactant × (coefficient of product ÷ coefficient of reactant)
Step 3: mass = moles × molar mass
Common Molar Masses
| Substance | Formula | Molar Mass (g/mol) |
|---|---|---|
| Hydrogen | H₂ | 2.02 |
| Oxygen | O₂ | 32.00 |
| Water | H₂O | 18.02 |
| Carbon dioxide | CO₂ | 44.01 |
| Sodium chloride | NaCl | 58.44 |
| Iron | Fe | 55.85 |
| Calcium carbonate | CaCO₃ | 100.09 |
Percent Yield
In real experiments, reactions rarely go to 100% completion:
Percent yield = (Actual yield ÷ Theoretical yield) × 100%
Typical lab reactions yield 70–95%. A yield below 50% usually indicates a problem with technique or reaction conditions.
Limiting Reagent
When two or more reactants are present, the one that runs out first is the limiting reagent. It limits the amount of product that can form. The other reactant(s) are in excess — some remains unused.