Stoichiometry and Mole Ratios
Simple Explanation
Stoichiometry uses the balanced equation's coefficients as mole ratios, letting you calculate how many moles of one substance react with or produce another.
Why Do We Need It?
This is how chemists predict exactly how much product a reaction will make, or how much reactant is needed, before ever running the experiment.
Worked Example
Use a mole ratio to find product amount
In the reaction 2H₂ + O₂ → 2H₂O, how many moles of water are produced from 5 moles of hydrogen (H₂)?
Why Does This Work?
The coefficients in a balanced equation are already a mole ratio — they tell you exactly how many moles of each substance react or form for every mole of the others, so scaling that ratio by the amount you have gives the right answer.
Real-Life Example
Scaling up a recipe
A balanced equation is like a recipe: 2 parts flour to 1 part sugar always stays in that ratio, however much you bake.
Just as doubling a recipe doubles every ingredient in the same ratio, doubling the moles of one reactant scales every other substance in the equation by the same factor.
Practice
In the reaction N₂ + 3H₂ → 2NH₃, how many moles of NH₃ are produced from 6 moles of H₂?
MediumCommon mistake
Using the ratio of the substances' masses instead of their mole-ratio coefficients — stoichiometry works in moles, not grams, until a final conversion step.
Quick Review
- Coefficients in a balanced equation give the mole ratio between substances.
- Scale the ratio to match the amount of a known substance.
- Convert to mass afterwards using n = m/M if needed.