⚗️ Stoichiometry and Limiting Reagents

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 2 independent reference calculations · how PhDino checks its numbers

Finding which reactant runs out first, and how much product a reaction actually produces.

A balanced chemical equation is a recipe written in moles: the coefficients say what ratio of each reactant combines to form each product. Real reactions are almost never mixed in exactly that ratio, though — one reactant usually runs out first. That one is the limiting reagent, and it's what actually caps how much product can form, no matter how much of the other reactant is sitting around unused.

A convenient way to find the limiting reagent is to divide each reactant's moles by its own coefficient in the balanced equation — whichever gives the smaller number is limiting, since that's the reactant that "runs out" of reaction cycles first. That same smallest number, multiplied by the product's coefficient, gives the moles of product formed.

Key formula

Reaction units = moles of reactant ÷ its equation coefficient
Limiting reagent = whichever reactant gives the smaller reaction-units value
Product moles = limiting reaction units × product coefficient

Notes & limitations

• This calculates a theoretical maximum assuming the reaction goes to completion — real reactions rarely do, which is exactly what percent yield (comparing an actual lab result to this theoretical number) measures separately.
• For more than two reactants, the same idea extends directly: compute reaction units for every reactant and take the smallest.