Chemistry
Stoichiometry — quick study summary
Stoichiometry is the maths of chemical reactions: using a balanced equation to work out how much of one substance reacts with or produces another. The bridge between substances is moles — convert grams to moles, use the equation's coefficients as a mole ratio, then convert back to grams (or litres for gases at STP). The limiting reagent is the one that runs out first and caps how much product forms; the excess reagent is what's left over.
Key points
- Mole = 6.022 × 10²³ particles (Avogadro's number)
- Moles = mass ÷ molar mass; for gases at STP: moles = volume ÷ 22.4 L
- Coefficients in a balanced equation give the mole ratio of substances
- Limiting reagent runs out first and determines maximum product
- % yield = (actual ÷ theoretical) × 100
Practice quiz
Click each question to reveal the answer.
1. How many moles are in 36 g of water (H₂O, molar mass 18 g/mol)?
- 0.5 moles
- 1 mole
- 2 moles
- 36 moles
Answer: 2 moles
moles = mass ÷ molar mass = 36 ÷ 18 = 2 mol.
2. In the reaction 2H₂ + O₂ → 2H₂O, how many moles of water form from 4 moles of H₂ (excess O₂)?
Answer: 4 moles
Mole ratio H₂ : H₂O is 2 : 2 = 1 : 1, so 4 moles of H₂ produce 4 moles of H₂O.
3. If you mix 3 mol H₂ with 2 mol O₂, which is the limiting reagent in 2H₂ + O₂ → 2H₂O?
Answer: H₂
3 mol H₂ needs 1.5 mol O₂ — you have 2 mol so O₂ is in excess. H₂ runs out first.
Last reviewed: May 2026