Stoichiometry, Explained From the Ground Up
Updated 2026-07-12
Stoichiometry is the part of chemistry that uses a balanced chemical equation to calculate the amounts of reactants and products. The coefficients in the balanced equation give mole ratios, which act as conversion factors between substances.
Stoichiometry is really unit conversion driven by the balanced equation. The coefficients tell you how many moles of one substance correspond to another, and that ratio is the bridge for every calculation.
The mole ratio is everything
In a balanced equation, the coefficients are a recipe in moles. For 2 H2 + O2 giving 2 H2O, the ratio of hydrogen to water is 2 to 2, or 1 to 1. That ratio is what converts moles of one substance into moles of another.
Balance first, always
Stoichiometry only works on a balanced equation. If the coefficients are wrong, every ratio built from them is wrong, so balancing is step zero of any stoichiometry problem.
Worked example
For 2 H2 + O2 giving 2 H2O, how many moles of water form from 4 moles of hydrogen (with excess oxygen)?
- Check the equation is balanced. It is: 4 H and 2 O on each side.
- Read the mole ratio of hydrogen to water: 2 to 2, which simplifies to 1 to 1.
- Apply it to 4 moles of hydrogen.
Answer: 4 moles of water.
The common mistake
Wrong: Using the coefficients from an unbalanced equation.
Fix: Every mole ratio comes from the balanced equation. Balance it first, then read the coefficients, or every downstream number will be off.
Learn Stoichiometry live, not just read about it
Dudely builds this exact concept on a whiteboard, explains every step, and checks you understand before moving on.
Learn it live, freeRelated concepts
Frequently asked questions
What is stoichiometry?
It is the use of a balanced chemical equation to calculate the amounts of reactants and products. The coefficients give mole ratios that convert between substances.
What is a mole ratio?
It is the ratio of the coefficients of two substances in a balanced equation. It acts as a conversion factor to go from moles of one substance to moles of another.