Limiting Reagents Explained

The limiting reagent is the reactant that runs out first. It controls how much product can form — the theoretical yield — no matter how much of the other reactants you have.

The method

1. Balance the equation so the mole ratios are correct.
2. Convert each given reactant to moles with n = m ÷ M.
3. Work out how much product each reactant could make on its own, using its mole ratio to the product.
4. The smaller answer is the theoretical yield, and the reactant that produced it is the limiting reagent. The other reactant is in excess.

Worked example

N₂ + 3 H₂ → 2 NH₃. Mix 28.0 g N₂ and 6.00 g H₂.

Hydrogen makes less ammonia, so H₂ is the limiting reagent and the theoretical yield is 1.98 mol × 17.03 g/mol ≈ 33.8 g NH₃.

Percent yield

Once you have the theoretical yield, percent yield compares it to what was actually obtained:

Find the molar masses you need with the Molar Mass Calculator, and review the general method in How to Approach Stoichiometry.