4.5 Stoichiometry

Mole ratios connect reactants and products

Stoichiometry uses the balanced chemical equation's coefficients as mole ratios to relate amounts of reactants and products. Convert given quantities to moles, apply the mole ratio from the balanced equation, then convert to the desired unit (grams, particles, liters of gas, or molarity). The balanced equation must always be correct before starting calculations.

Limiting and excess reactants

When reactants aren't in exact stoichiometric proportions, one runs out first — the limiting reactant — and determines the maximum amount of product formed. The other reactant is in excess. To find the limiting reactant, calculate moles of product possible from each reactant; the smaller value identifies the limiting reactant and the actual yield.

Percent yield

Theoretical yield is the maximum product predicted from stoichiometry, while actual yield is what's measured experimentally. Percent yield = (actual yield ÷ theoretical yield) × 100%. Yields below 100% arise from side reactions, incomplete reactions, or loss of product during handling and purification.

Key terms

Stoichiometry

The calculation of quantitative relationships between reactants and products using mole ratios from a balanced equation.

Limiting reactant

The reactant that is completely used up first, determining the maximum amount of product that can form.

Excess reactant

The reactant that remains after the limiting reactant is completely consumed.

Theoretical yield

The maximum amount of product predicted by stoichiometric calculations based on the limiting reactant.

Percent yield formula

Percent yield = (actual yield ÷ theoretical yield) × 100%

Try an AP-style question

In 2H₂ + O₂ → 2H₂O, what mass of water forms from 4.0 g of H₂ (2.0 mol) with excess O₂?

1. 36 g
2. 4.0 g
3. 18 g
4. 72 g