Stoichiometry: Recipe Math
Balanced equation + mole bridge = the power to predict exactly how many grams come out — and which ingredient runs out first.
Builds on: 2.1 Chemical Reactions & Balancing2.2 The Mole: Chemistry's Dozen
The three-step machine
Stoichiometry (from Greek stoicheion, element) answers questions like: “How much CO₂ does burning 10 g of methane release?” The machine has three gears, always the same:
- Grams → moles for what you know (n = m/M).
- Moles → moles via the balanced equation’s coefficient ratio.
- Moles → grams for what you want (m = n·M).
Notice the shape of the trick: never compare grams to grams directly. Grams of different substances aren’t comparable — moles are. Convert in, ratio across, convert out.
The limiting reagent
Recipes fail realistically: you have flour for 30 pancakes but eggs for 12 — you get 12 pancakes and leftover flour. Reactions are identical. Whichever reactant runs out first is the limiting reagent; it alone decides the yield, and the excess of the other just sits there. To find it, convert both reactants to moles, divide each by its coefficient, and the smaller quotient loses.
Industrial chemistry is stoichiometry with money attached: feed a reactor the wrong ratio and you either waste expensive reagent or leave product unmade. The same math sizes the CO₂ balloon in your Unit 4 kitchen capstone — baking soda and vinegar in the right proportion, nothing wasted.
⚗️ Lab — The Limiting-Reagent Mixer
Mix hydrogen and oxygen in any amounts; the reaction 2 H₂ + O₂ → 2 H₂O takes what it can.
- Set 4 mol H₂ and 4 mol O₂ — oxygen is left over. Why?
- Find the perfect ratio where both bars empty together.
- Double only the oxygen — does more O₂ make more water?
Work it by hand — 0 / 4
No multiple choice here: compute the value and type it. Suffixes like 2.5k, 20m or 100µ are understood; answers within ±2% count.