What the limiting reagent is
In a reaction, the reactants are used up in the ratio set by the balanced equation. The limiting reagent is the one that runs out first; it decides how much product can form. The others are in excess, and some of each is left over.
The method
- Balance the equation.
- Convert the mass of each reactant to moles: moles = mass ÷ molar mass.
- Divide each reactant's moles by its coefficient in the equation.
- The reactant with the smallest result is the limiting reagent.
A worked example
Hydrogen burns in oxygen: 2 H2 + O2 → 2 H2O. You have 4.0 g of H2 and 16.0 g of O2.
- H2: 4.0 ÷ 2.016 = 1.98 mol, ÷ 2 = 0.99
- O2: 16.0 ÷ 32.00 = 0.50 mol, ÷ 1 = 0.50
Oxygen gives the smaller number, so O2 is the limiting reagent, even though there is more of it by mass.
Theoretical yield and leftovers
The 0.50 mol of O2 makes 2 × 0.50 = 1.0 mol of water, which is 1.0 × 18.02 = 18.0 g, the theoretical yield. It uses 1.0 mol of H2, 2.02 g, so 4.0 − 2.02 = 1.98 g of hydrogen is left over.
The mass you actually collect divided by the theoretical yield is the percent yield.
Common mistakes
- Comparing masses instead of moles.
- Comparing moles without dividing by the coefficients.
- Using an unbalanced equation.
Calculators
The Limiting Reagent Calculator does all of this from formulas and masses. Balance the equation first with the Chemical Equation Balancer, look up molar masses with the Molar Mass Calculator, and compare your result with the Percent Yield Calculator.