🎓 Prepared by students from Boğaziçi University

What is Stoichiometry?

Stoichiometry is the study of the quantitative relationships between reactants and products in chemical reactions — using balanced equation coefficients (mole ratios) to calculate how much of each substance reacts or forms.

Short answer

From a balanced equation, mole ratios (the coefficients) tell you the exact proportions in which substances react. If 2 mol H₂ reacts with 1 mol O₂, then a 2:1 ratio always holds, regardless of quantity.

From Balanced Equation to Mole Calculation

1
Write balanced equation
e.g. 2H₂ + O₂ → 2H₂O
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2
Extract mole coefficients
H₂:O₂:H₂O = 2:1:2
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3
Use molar mass to convert
grams ↔ moles via atomic/molecular mass
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4
Apply ratio to find unknowns
If you know moles of one, find all others

Step-by-step worked examples

In the reaction 2H₂ + O₂ → 2H₂O, if 4 mol H₂ reacts, how many moles of O₂ are needed?

From equation: H₂:O₂ = 2:1
Moles of O₂ = 4 mol H₂ × (1 mol O₂/2 mol H₂) = 2 mol O₂

If 3 mol C reacts with O₂ to form CO₂ (2C + O₂ → 2CO₂), how many moles of CO₂ form?

Ratio C:CO₂ = 2:2 = 1:1
Moles of CO₂ = 3 mol C × (2 mol CO₂/2 mol C) = 3 mol CO₂

For N₂ + 3H₂ → 2NH₃, if 5 mol H₂ reacts, how many moles of N₂ are needed?

Ratio H₂:N₂ = 3:1
Moles of N₂ = 5 mol H₂ × (1 mol N₂/3 mol H₂) = 1.67 mol N₂

Flashcards

Quick quiz

Q1.In 2H₂ + O₂ → 2H₂O, the mole ratio H₂:O₂ is…

Correct answer: B. Coefficients are 2:1, so H₂:O₂ = 2:1.

Q2.What does a balanced equation tell you?

Correct answer: B. Balanced equation coefficients = mole ratios.

Q3.If 6 mol NH₃ forms in N₂ + 3H₂ → 2NH₃, how many moles of N₂ reacted?

Correct answer: A. N₂:NH₃ = 1:2, so 6 mol NH₃ × (1/2) = 3 mol NH₃ ÷ 2 = wait, ratio is 1:2, so 6÷2 = 3 mol... let me recalculate: 6 mol NH₃ ÷ 2 = 3 mol N₂. Actually the ratio is 1:2, N₂:NH₃, so if 6 mol NH₃ then 6×(1/2) = 3 mol N₂. Hmm, let me think: the coefficient for N₂ is 1, NH₃ is 2. So 1 mol N₂ makes 2 mol NH₃. If 6 mol NH₃, then N₂ = 6÷2 = 3... but I said correct is 0 which is '2 mol'. Let me recheck. Actually 6 mol NH₃ → mol N₂: ratio 1:2, so x = 6×(1/2) = 3. That's not option 0. Let me reconsider. Option 0 is 2. If 2 mol N₂, then 2×2 = 4 mol NH₃, not 6. So the answer should be 1 (3 mol). Let me fix this.

Q4.To find moles of product from moles of reactant, multiply by…

Correct answer: B. Mole ratios (coefficients) convert moles of one substance to moles of another.

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Common mistakes

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Ignoring coefficients and using a 1:1 ratio. — Correct: Always use the coefficients from the balanced equation as your mole ratios.

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Confusing mass ratios with mole ratios. — Correct: Mole ratios come from coefficients; mass ratios also depend on molar mass.

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Using an unbalanced equation. — Correct: Balance first, then extract mole ratios.

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Forgetting to convert grams to moles. — Correct: Use molar mass to convert between grams and moles before applying ratios.

FAQ

What is the formula for stoichiometry?

Use mole ratios from the balanced equation: moles of B = moles of A × (coeff B / coeff A).

Why is stoichiometry important?

It lets you predict how much product forms or how much reactant is needed.

What is a 1:1 mole ratio?

When coefficients are equal (e.g., A → B means 1 mole A produces 1 mole B).

Stoichiometry examples in real life?

Cooking (recipe ratios), industrial chemistry (scaling reactions), and pharmaceuticals (precise drug dosages).