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Stoichiometry

A recipe tells you two eggs per cup of flour, not two grams per gram. Reactions are the same: the balanced equation gives a ratio in moles, and finding the limiting reactant means comparing moles, never grams.

§1

The mole ratio runs everything.

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Stoichiometry uses the balanced equation's coefficients as a mole ratio to relate amounts of reactants and products. For 2H₂ + O₂ → 2H₂O, hydrogen and oxygen react in a 2:1 mole ratio.

To use it, convert given amounts to moles first (grams alone are misleading because different substances have different molar masses). Then apply the mole ratio to find any other amount.

When two reactant amounts are given, one is the limiting reactant — it runs out first and caps the product. The other is in excess and is left over. You find the limiter by comparing how much product each reactant could make.

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
Fig. 4.5.1 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.
§2

Finding the limiting reactant.

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Everything happens in moles. Convert first, then compare.

  1. Convert each reactant to moles. Divide each mass by its molar mass. Never compare reactants by grams.
  2. Use the mole ratio. Compare how much product each reactant could produce using the balanced coefficients.
  3. Pick the limiter. The reactant that makes the least product is limiting; it runs out first.
  4. Find product and leftover. The limiting reactant sets the product amount; the other reactant is in excess and partly left over.
§3

The pieces you'll meet.

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A few terms, all in moles.

mole ratio
Mole ratio
The balanced coefficients relating amounts.
limiting
Limiting reactant
Runs out first; caps the product formed.
excess
Excess reactant
Left over after the limiting reactant is used up.
convert
Grams to moles
Always convert before comparing reactant amounts.
product
Product amount
Set by the limiting reactant, via the mole ratio.
coefficients
Coefficients
The numbers that define the mole ratio.
§4

Worked example: which reactant limits?

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Question. For N₂ + 3H₂ → 2NH₃, you have 2 mol N₂ and 3 mol H₂. Which is limiting, and how much NH₃ forms?

Nitrogen's capacity. 2 mol N₂ could make 2 × 2 = 4 mol NH₃ (ratio 1 N₂ : 2 NH₃).

Hydrogen's capacity. 3 mol H₂ could make 3 × (2/3) = 2 mol NH₃ (ratio 3 H₂ : 2 NH₃).

Conclusion. Hydrogen makes less product (2 mol vs 4 mol), so H₂ is limiting and 2 mol NH₃ forms. Nitrogen is in excess — even though there are fewer moles of N₂, the mole ratio, not the raw count, decides.

§5

Mistakes that cost real points.

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Pitfall · 01

"The reactant with fewer grams is the limiting reactant."

Grams do not decide the limiter, because substances have different molar masses. You must convert to moles and compare via the mole ratio. A reactant with more grams can still be limiting if its molar mass is large.

Fix. Convert every reactant to moles first, then compare how much product each can make. Never judge the limiter by grams.

Pitfall · 02

"Equal moles of two reactants means neither is limiting."

Equal moles are not enough; the balanced ratio matters. For N₂ + 3H₂, equal moles of N₂ and H₂ leaves H₂ limiting, because the reaction needs three times as much H₂ as N₂. The coefficients, not equality, decide.

Fix. Compare reactants against the balanced mole ratio, not against each other one-to-one.

Pitfall · 03

"After a reaction, both reactants are completely used up."

Only the limiting reactant is fully consumed; the excess reactant is partly left over. Reactions rarely use up both perfectly, because that requires the amounts to match the exact mole ratio.

Fix. Expect leftover excess reactant. The limiting reactant runs out; the other remains in part.

§6

Skill Check.

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