Stoichiometry
Reference for stoichiometry: use mole ratios from balanced equations to calculate masses.
Covers molar mass, limiting reagent, percent yield, and examples.
Need to calculate, not just reference? Use the interactive version.
Open Stoichiometry Calculator →
The Formula
Moles of B = Moles of A × (Coefficient of B / Coefficient of A)
Stoichiometry uses the balanced chemical equation to convert between amounts of reactants and products. The coefficients in the equation give the mole ratios.
Variables
| Symbol | Meaning |
|---|---|
| Moles of A | Known amount of one substance |
| Moles of B | Unknown amount of another substance |
| Coefficient | Number in front of a substance in the balanced equation |
Step-by-Step Process
- Write and balance the chemical equation
- Convert given amount to moles (using molar mass if given grams)
- Use the mole ratio from the balanced equation
- Convert moles to desired units (grams, liters, particles)
Example 1
2H₂ + O₂ → 2H₂O. How many moles of water from 3 moles of H₂?
Mole ratio: 2 mol H₂ produces 2 mol H₂O (ratio = 1:1)
Moles of H₂O = 3 × (2/2)
= 3 moles of H₂O
Example 2
How many grams of O₂ needed to react with 10 g of H₂? (2H₂ + O₂ → 2H₂O)
Moles of H₂ = 10 g / 2.016 g/mol = 4.96 mol
Mole ratio: 2 mol H₂ : 1 mol O₂
Moles of O₂ = 4.96 × (1/2) = 2.48 mol
Grams of O₂ = 2.48 × 32.00 g/mol
= 79.4 g of O₂
When to Use It
Use stoichiometry when:
- Predicting how much product a reaction will yield
- Determining how much reactant is needed
- Finding the limiting reagent in a reaction
- Scaling up reactions for industrial or lab purposes
Key Notes
- Mole ratios from balanced equations: Coefficients in a balanced equation give the exact molar ratios of reactants and products. In 2H₂ + O₂ → 2H₂O, 2 moles of H₂ react with 1 mole of O₂ to produce 2 moles of H₂O — regardless of the actual quantities used.
- The limiting reagent determines yield: The reactant that runs out first limits how much product can form. The other reactant is in excess. Identify the limiting reagent by converting all reactant quantities to moles and comparing to stoichiometric ratios.
- Percent yield: (actual / theoretical) × 100: Theoretical yield is the maximum from stoichiometry; actual yield is what you measure in the lab. Values under 100% reflect incomplete reaction, side reactions, handling losses, or measurement error. Values over 100% indicate impurities or errors.
- Molar mass conversion chain: grams → moles (÷ molar mass) → moles product (× coefficient ratio) → grams product (× molar mass). Every stoichiometry calculation follows this exact pathway. Keep track of units at every step.
- Applications: Stoichiometry is used in industrial chemical production (scaling reactions), pharmaceutical synthesis (maximizing yield, minimizing waste), food science (recipe formulation), environmental monitoring (emission calculations), and analytical chemistry (titration endpoint calculations).