How to Calculate Stoichiometry
Stoichiometry uses balanced chemical equations to calculate the amounts of reactants consumed and products formed in a chemical reaction. It's the quantitative backbone of chemistry—essential for labs, industrial chemistry, and pharmaceuticals.
The Mole Road Map
Mass → (÷ molar mass) → Moles → (× mole ratio) → Moles → (× molar mass) → Mass
Step-by-Step Example
Reaction: 2H₂ + O₂ → 2H₂O How many grams of water form when 4 grams of hydrogen reacts completely?
Step 1: Convert grams of H₂ to moles. Molar mass of H₂ = 2 g/mol 4 g ÷ 2 g/mol = 2 moles H₂
Step 2: Use mole ratio from balanced equation. 2 mol H₂ : 2 mol H₂O → ratio = 1:1 Moles of H₂O = 2 × (2/2) = 2 moles H₂O
Step 3: Convert moles of water to grams. Molar mass of H₂O = 18 g/mol 2 moles × 18 g/mol = 36 grams of water
Finding the Limiting Reagent
The limiting reagent is the reactant that runs out first, determining how much product forms.
Example: You have 3 mol H₂ and 2 mol O₂. Which is limiting?
- H₂ can make: 3 × (2/2) = 3 mol H₂O
- O₂ can make: 2 × (2/1) = 4 mol H₂O
H₂ produces less product → H₂ is the limiting reagent. Maximum product = 3 mol H₂O = 54 grams.
Theoretical vs. Actual Yield
% Yield = (Actual yield / Theoretical yield) × 100%
If you actually collected 50 g of water: % yield = 50/54 × 100 = 92.6%
Use our stoichiometry calculator for any balanced chemical equation.