The limiting reactant (or limiting reagent) is the reactant that runs out first in a chemical reaction, determining the maximum amount of product that can form. The other reactant(s) are in excess.
For each reactant, calculate:
Moles of Product = (moles reactant) × (mole ratio)
Limiting reactant = produces the LEAST product
Making sandwiches: 2 slices bread + 1 slice cheese → 1 sandwich
You have: 10 slices bread and 3 slices cheese
Same concept applies to chemical reactions!
Make sure coefficients are correct - they give mole ratios
Use n = mass / molar mass for each reactant
For each reactant, use stoichiometry to find how much product it could make
nproduct = nreactant × (coefficientproduct / coefficientreactant)
Whichever reactant produces the SMALLEST amount of product is limiting
The limiting reactant determines theoretical yield
4 Al + 3 O₂ → 2 Al₂O₃
Molar masses: Al = 27.0 g/mol, O₂ = 32.0 g/mol, Al₂O₃ = 102.0 g/mol
nAl = 10.0 g / 27.0 g/mol = 0.370 mol Al
nO₂ = 19.0 g / 32.0 g/mol = 0.594 mol O₂
Mole ratio: 2 mol Al₂O₃ / 4 mol Al = 1/2
nAl₂O₃ from Al = 0.370 mol Al × (2 mol Al₂O₃ / 4 mol Al)
= 0.185 mol Al₂O₃
Mole ratio: 2 mol Al₂O₃ / 3 mol O₂ = 2/3
nAl₂O₃ from O₂ = 0.594 mol O₂ × (2 mol Al₂O₃ / 3 mol O₂)
= 0.396 mol Al₂O₃
Al produces: 0.185 mol Al₂O₃ ← SMALLER
O₂ produces: 0.396 mol Al₂O₃
Al is the limiting reactant!
It produces less product, so it runs out first. O₂ is in excess.
Use the limiting reactant result:
massAl₂O₃ = 0.185 mol × 102.0 g/mol
= 18.9 g Al₂O₃
Final Answers:
You can't just compare masses! Must convert to moles and use mole ratios from balanced equation.
The reactant with fewer moles isn't always limiting! Must account for stoichiometry. 1 mol of a 1:10 reactant can still be in excess.
Mole ratios come from coefficients - equation MUST be balanced first!
Always use the limiting reactant to calculate theoretical yield. The excess reactant has leftover!
The reactant that gets completely used up first in a reaction, limiting the amount of product that can form. Other reactants are in excess.
Calculate how much product each reactant could make. Whichever produces the LEAST amount of product is the limiting reactant.
Because reactions use reactants in specific ratios! 2H₂ + O₂ → 2H₂O needs 2:1 ratio. Even if you have 10 mol H₂ and 3 mol O₂, you must check stoichiometry.
Some remains unreacted. You can calculate how much was used and subtract from the starting amount to find leftover excess.
No! Limiting reactant is a reactant. Theoretical yield is the maximum product amount, calculated FROM the limiting reactant.