limiting reagent

Meaning: When a reaction runs to completion, the reactant that runs out first is the limiting reagent. Once it is gone, no more product can form, so it fixes the theoretical yield. Every other reactant is in excess, and some of it remains afterward.

Equation: limiting reagent = the reactant with the smallest n/ν, where n is the amount of reactant (mol) and ν is its coefficient in the balanced equation (dimensionless). Theoretical yield of product (mol) = n(limiting) × (coefficient of product / coefficient of limiting reagent).

On the MCAT: A passage gives masses or volumes of two reactants. Convert each to moles with molar mass, divide by the coefficients, pick the smallest, then find the theoretical yield or the amount of excess left over. Percent yield = (actual yield / theoretical yield) × 100%. Conceptual questions ask what happens to product if you add more of one reactant: adding the limiting reagent increases product, while adding more of an excess reactant does not.

Common confusion: Students pick the reactant with the fewest grams or fewest moles. That ignores the coefficients. In 2H₂ + O₂ → 2H₂O with 3 mol H₂ and 2 mol O₂, O₂ has fewer moles, but H₂ is limiting: 3/2 = 1.5 is smaller than 2/1 = 2.