What is Stoichiometry?
Stoichiometry is the study of the quantitative relationships between reactants and products in chemical reactions — it tells us how many molecules of A react with B to form C. A balanced chemical equation is the foundation of all stoichiometric calculations.
Stoichiometry uses mole ratios from a balanced equation to predict how much product forms or how much reactant is needed. For example, in 2H₂ + O₂ → 2H₂O, 2 moles of H₂ react with 1 mole of O₂ to make 2 moles of water.
- 1↓Write balanced equationEnsure atoms balance on both sides.
- 2↓Convert given mass to molesUse molar mass: moles = mass (g) / M (g/mol).
- 3↓Use mole ratioMultiply moles by stoichiometric coefficient ratio.
- 4Convert to desired unitMoles to mass, volume (at STP), or particles as needed.
Step-by-step worked examples
In the reaction 2H₂ + O₂ → 2H₂O, if 4 moles of H₂ react with O₂, how many moles of H₂O are produced?
From the equation: 2 mol H₂ → 2 mol H₂O (1:1 ratio) 4 mol H₂ × (2 mol H₂O / 2 mol H₂) = 4 mol H₂O
Given the reaction C + O₂ → CO₂, how many grams of O₂ are needed to burn 12 g of carbon (C, M=12 g/mol)?
Moles of C = 12 g / 12 g/mol = 1 mol Mole ratio: 1 mol C : 1 mol O₂ 1 mol C × (1 mol O₂ / 1 mol C) = 1 mol O₂ Mass O₂ = 1 mol × 32 g/mol = 32 g
In the reaction 2NH₃ → N₂ + 3H₂, if 6 moles of NH₃ decompose, how many moles of H₂ form?
Mole ratio: 2 mol NH₃ → 3 mol H₂ 6 mol NH₃ × (3 mol H₂ / 2 mol NH₃) = 9 mol H₂
Flashcards
Quick quiz
Q1.In 3O₂ → 2O₃, if 6 moles of O₂ react, how many moles of O₃ form?
Q2.To find moles from mass, you use…
Q3.In a reaction, stoichiometric coefficients represent…
Q4.If the equation 2A + B → 2C, and 3 mol of A reacts, B required is…
The full card deck, worked steps and AI-tutor support for “What is Stoichiometry?” are in Notek — study by hand before your exam.
Common mistakes
Using coefficients as mass ratios instead of mole ratios. — Correct: Coefficients are mole ratios; convert to mass using molar mass.
Forgetting to balance the equation before stoichiometry. — Correct: Always balance first — wrong equation → wrong answer.
Confusing mass with moles. — Correct: Mass (g) and moles are different; convert using M = m/n.
Ignoring what the question asks for. — Correct: Read carefully — the answer may need to be in grams, moles or volume.
FAQ
What is stoichiometry?
The quantitative analysis of chemical reactions — it uses balanced equations to relate the amounts of reactants and products.
What does a balanced equation tell us?
It shows the mole ratio of each reactant and product — how many moles of each substance participate in the reaction.
What is a limiting reactant?
The reactant that runs out first in a reaction, determining how much product can form.
How do you solve a stoichiometry problem?
1) Balance the equation. 2) Convert mass to moles. 3) Use mole ratio. 4) Convert to desired unit.