🎓 Prepared by students from Boğaziçi University

What Is Stoichiometry?

Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction, based on balanced equations. It allows calculation of moles, mass, and volumes of reactants or products.

Short answer

Stoichiometry uses the coefficients in balanced equations to relate moles of reactants and products. Molar ratios allow conversions: moles A → moles B using the ratio of coefficients.

Stoichiometry workflow: balanced equation to moles to grams

1
Write balanced equation
Coefficients show molar ratios
↓
2
Convert grams to moles
Use molar mass (g/mol)
↓
3
Apply molar ratio
Multiply by coefficient ratio
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4
Convert moles to grams
Multiply by molar mass of product

Try it: interactive calculator

Moles of product B

1mol

= (2 / 2) * 1

Moles of reactant A2 molCoefficient of A in equation2 Coefficient of B (product) in equation1

Step-by-step worked examples

Balanced equation: 2H₂ + O₂ → 2H₂O. If 4 moles of H₂ react, how many moles of H₂O form?

Molar ratio from equation: 2 mol H₂ : 2 mol H₂O = 1:1
Moles H₂O = 4 mol H₂ × (2 mol H₂O / 2 mol H₂) = 4 mol H₂O

From 2C + O₂ → 2CO, if 12 g of C (molar mass 12 g/mol) reacts, how many moles of CO form?

Moles C = 12 g ÷ 12 g/mol = 1 mol C
Molar ratio: 2 mol C : 2 mol CO = 1:1
Moles CO = 1 mol C × (2 mol CO / 2 mol C) = 1 mol CO

From 4Fe + 3O₂ → 2Fe₂O₃, if 2 moles of Fe₂O₃ form, how many moles of O₂ were used?

Molar ratio: 3 mol O₂ : 2 mol Fe₂O₃
Moles O₂ = 2 mol Fe₂O₃ × (3 mol O₂ / 2 mol Fe₂O₃) = 3 mol O₂

Flashcards

Quick quiz

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

Correct answer: B. From the balanced equation: 2 mol H₂ produces 2 mol H₂O, so the ratio is 1:1.

Q2.If 3 moles of A react via 2A → 3B, moles of B formed?

Correct answer: C. 3 mol A × (3 mol B / 2 mol A) = 4.5 mol B

Q3.Converting grams to moles requires…

Correct answer: B. Molar mass (g/mol) is used to convert between grams and moles.

Q4.The limiting reactant is…

Correct answer: B. The limiting reactant determines how much product can form because it is consumed first.

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Common mistakes

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Ignoring coefficients when calculating molar ratios. — Correct: Coefficients in the balanced equation are essential — they define the molar ratio.

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Confusing grams and moles. — Correct: Moles are counted; grams are mass. Convert using molar mass (g/mol).

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Assuming all reactants are consumed equally. — Correct: The limiting reactant runs out first; others remain in excess.

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Applying stoichiometry to unbalanced equations. — Correct: Always balance the equation first — unbalanced coefficients are meaningless.

FAQ

What is a molar ratio?

The ratio of coefficients in a balanced equation, used to relate moles of reactants to products.

How do you find the limiting reactant?

Calculate moles of each reactant. The one requiring the fewest moles of product is limiting.

What is theoretical yield?

The maximum product amount calculated from the limiting reactant using stoichiometry.

Why is stoichiometry important?

It predicts reactant and product amounts, ensuring efficient resource use and complete reactions.