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Stoichiometry

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Stoichiometry uses the balanced equation's coefficients as a mole ratio to relate amounts of reactants and products. To find how much product forms, convert everything to moles, apply the ratio, and identify the limiting reactant — the one that runs out first and caps the product.

UNIT 4 TOPIC 4.5 • STOICHIOMETRY REACTION SCALE BALANCED EQUATION SETS THE RATIO 2H₂ + O₂ → 2H₂O mole ratio: 2 : 1 : 2 INPUTS Start: 5 mol H₂ 2 mol O₂ H H H H H H H H H H O O O O LIMITING REAGENT CHECK O₂ controls the run 2 mol O₂ needs 4 mol H₂ H₂ available = 5 mol H₂ leftover = 1 mol used: 4 mol H₂ and 2 mol O₂ PRODUCTS 2 mol O₂ → 4 mol H₂O O H H O H H O H H O H H STOICHIOMETRY PATH known amount → mole ratio from coefficients → unknown amount Coefficients compare moles or particles, not grams directly. Atoms are conserved, so product amounts are locked to reactant amounts. AP Chemistry · Unit 4 · Chemical Reactions
The balanced equation sets the mole ratio. For 2H₂ + O₂ → 2H₂O, hydrogen and oxygen react 2:1. That ratio, applied in moles, determines the limiting reactant and how much product forms — grams must be converted to moles first.
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The traps come from skipping the conversion to moles: picking the limiting reactant by grams instead of moles, assuming equal moles means no limiter, or thinking both reactants are fully used up. The mole ratio, not the masses, governs everything.

The work

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Lesson
Stoichiometry ›

Coefficients give the mole ratio that sets the limiting reactant and the product amount. The lesson converts to moles and finds the limiter, then closes with a ten-scenario check.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items spanning the Topic 4.5 misconceptions: picking the limiting reactant from grams, assuming equal moles has no limiter, expecting both reactants fully consumed, and thinking more excess makes more product.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions
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