Stop 7 of 12 · The path
Stoichiometry — how much
Moles from stop 5 plus equations from stop 6 give amounts: what runs out, what is left, what you should make.
Bench tool
What runs out? 2H₂ + O₂ → 2H₂O
Type what you have — the tool names the limiting reactant and counts the water:
The recipe: grams → moles → moles (ratio) → grams.
How it works
- Find the limiting reactant. The one that runs out first caps the product.
- Use the mole ratio. 2H₂ + O₂ → 2H₂O means 2 mol H₂ per 1 mol O₂.
- Compare to reality. Yield = actual ÷ expected × 100%.
Worked examples
10 g CaCO₃ → CaO. 10 ÷ 100 = 0.1 mol → 0.1 mol CaO = 5.6 g.
Excess and limiting. Lots of HCl, little Zn — zinc limits; hydrogen stops when zinc is gone.
80% yield. Expect 10 g, get 8 g → 80%. The rest stuck to glassware or side reactions.
Where it goes wrong
Grams do not ratio. Convert to moles before using the equation, convert back after.
Try it at home: sandwich limiting
Lunch maths. Lay out 6 bread slices and 2 cheese slices, then build the most 2-bread-1-cheese sandwiches you can. Bread piles up unused — the scarcer ingredient caps the product.
For teachers
Learners use the scarcest grams instead of the scarcest moles — probe with 4 mol H₂ + 1 mol O₂. Quickest check: the sandwich version — 6 bread, 2 cheese, how many sandwiches?