🎓 Prepared by students from Boğaziçi University

What Is Stoichiometry?

Stoichiometry is the branch of chemistry that uses the mole ratios in a balanced equation to calculate the amounts of reactants and products in a reaction. It lets chemists predict exactly how much product will form or how much reactant is needed.

Short answer

Stoichiometry is the calculation of quantitative relationships between reactants and products in a chemical reaction, using the coefficients of a balanced equation as mole ratios.

How to Solve a Stoichiometry Problem

1
Start with a balanced equation
Make sure the reaction equation is correctly balanced.
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2
Convert given mass to moles
Use n = m/M to find the moles of the given substance.
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3
Apply the mole ratio from coefficients
Use n_B = n_A × (b/a) to find moles of the desired substance.
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4
Convert moles to the requested unit
Convert moles of the target substance to mass or another unit if needed.

Try it: interactive calculator

Moles of substance B

4mol

= 2*(2/1)

Moles of given substance A2 molCoefficient of A in balanced equation1 Coefficient of B in balanced equation2

Step-by-step worked examples

In 2H2 + O2 → 2H2O, how many moles of H2O form from 3 mol H2?

Mole ratio H2:H2O = 2:2 = 1:1
n(H2O) = 3 mol × (2/2) = 3 mol
So 3 mol of H2 produce 3 mol of H2O

For N2 + 3H2 → 2NH3, how many moles of NH3 form from 6 mol H2?

Mole ratio H2:NH3 = 3:2
n(NH3) = 6 mol × (2/3) = 4 mol
So 6 mol H2 yields 4 mol NH3

How many grams of CO2 form when 4 mol C3H8 burns completely? (C3H8+5O2→3CO2+4H2O, M(CO2)=44 g/mol)

Mole ratio C3H8:CO2 = 1:3
n(CO2) = 4 mol × (3/1) = 12 mol
mass = n × M = 12 × 44 = 528 g CO2

Flashcards

Quick quiz

Q1.Stoichiometry calculations are based on:

Correct answer: B. The coefficients of a balanced equation give the mole ratios used in stoichiometry.

Q2.In 2H2 + O2 → 2H2O, moles of H2O from 5 mol H2:

Correct answer: B. Ratio H2:H2O = 2:2 = 1:1, so 5 mol H2 → 5 mol H2O.

Q3.What must you do before using mole ratios?

Correct answer: B. Mole ratios only come from the coefficients of a correctly balanced equation.

Q4.General stoichiometry order for a mass-to-mass problem:

Correct answer: A. Convert given mass to moles, apply the mole ratio, then convert to the target's mass.

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Common mistakes

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Using an unbalanced equation for mole ratios. — Correct: Always balance the equation first — ratios come from balanced coefficients.

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Skipping the conversion to moles before applying the ratio. — Correct: Convert mass to moles first (n=m/M), apply the ratio, then convert back if needed.

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Using the ratio of masses instead of moles. — Correct: Mole ratios come from coefficients, not from masses — always work in moles.

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Forgetting to convert the final answer to the requested unit. — Correct: Check what the question asks for (mass, moles, or particles) and convert the last step accordingly.

FAQ

What is stoichiometry?

It's using mole ratios from a balanced chemical equation to calculate quantities of reactants and products.

What is the stoichiometry formula?

n_B = n_A × (b/a), where a and b are the balanced-equation coefficients of substances A and B.

Can you give stoichiometry examples?

Finding grams of CO2 produced from burning a given mass of propane is a classic stoichiometry example.

How do you calculate stoichiometry step by step?

Convert given mass to moles, multiply by the mole ratio from the balanced equation, then convert to the units asked for.