Stoichiometry
Use mole ratios to calculate reactant and product amounts in chemical reactions.
Core chemistry skill.
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