percent yield

Meaning: Percent yield measures how efficiently a reaction turned reactants into the desired product. The theoretical yield is the maximum product, found from the balanced equation and the limiting reagent. The actual yield is the amount really collected. Actual yield is usually lower because the reaction may stop at equilibrium, side reactions use up reactants, or product is lost in transfer and purification. A value above 100% points to an impure or wet product, not extra product created.

Equation: percent yield = (actual yield ÷ theoretical yield) × 100%, where both yields must be in the same units (mol, or g converted with molar mass). The result is dimensionless.

On the MCAT: A passage gives masses of two reactants, a balanced equation and the mass of product collected. Convert to moles, identify the limiting reagent, use the mole ratio to get theoretical moles of product, convert to grams, then divide. For multistep syntheses, the overall yield is the product of the step yields (80% then 50% gives 40%). Questions may also ask how a side reaction or product loss changes the yield.

Common confusion: Students compute the theoretical yield from the reactant present in excess, or from whichever reactant is listed first. Only the limiting reagent sets the maximum. Also keep units matched: do not divide grams of product by moles of theoretical product.