Last updated: 2026-06-30

Understanding the limiting reagent

When two reactants are mixed, they are rarely present in the exact ratio the balanced equation calls for. One of them runs out first — the limiting reagent (also called the limiting reactant). It stops the reaction and fixes the maximum amount of product. Whatever is left of the other reactant is the excess.

The mole-ratio method

The trick is never to compare masses directly — atoms react in mole ratios, not gram ratios. The method has three steps:

1. Convert each reactant mass to moles by dividing by its molar mass.
2. Divide each mole amount by its coefficient in the balanced equation.
3. The reactant with the smaller ratio is the limiting reagent.

Worked example

For 2H₂ + O₂ → 2H₂O, mix 10 g of H₂ with 64 g of O₂.

1. Moles of H₂: 10 ÷ 2.016 = 4.96 mol; ratio = 4.96 ÷ 2 = 2.48
2. Moles of O₂: 64 ÷ 32 = 2.00 mol; ratio = 2.00 ÷ 1 = 2.00
3. O₂ has the smaller ratio, so O₂ is the limiting reagent.

How much excess is left over

Once the limiting reagent is known, the reaction runs to the extent it allows. The moles of the excess reactant consumed equal the limiting ratio times the excess coefficient. In the example, the reaction extent is 2.00, so H₂ consumed = 2.00 × 2 = 4.00 mol. Starting from 4.96 mol, that leaves 0.96 mol of H₂, or about 1.94 g unreacted.

Common reactions

From limiting reagent to yield

The limiting reagent sets the ceiling on product, so it is always the basis for a theoretical-yield calculation. Find the limiting reagent first, compute the theoretical yield from it, then compare against the product you actually recover to get the percent yield.