Reaction Yield and Percent Yield

Theoretical yield is the maximum mass of product possible from complete consumption of the limiting reactant. Convert each reactant to moles, use equation coefficients to find moles of product, then multiply by the product's molar mass. Decomposing calcium carbonate CaCO₃ (100.09 g/mol) to calcium oxide and CO₂: 10.0 g is 0.0999 mol; if the equation gives 1:1 product, expect about 5.60 g CaO — but verify the balanced equation and product identity first.

Actual yield is what you measure after isolation (filtered, dried, weighed). Side reactions, incomplete conversion, transfer losses, and impure reagents reduce actual yield below theoretical. Preparing aspirin C₉H₈O₄ (180.16 g/mol) from salicylic acid C₇H₆O₃ often gives percent yields well below 100% in student labs — that is normal, not necessarily a calculation error.

Percent yield = (actual yield ÷ theoretical yield) × 100%. Both masses must refer to the same product. If theoretical yield used silver chloride AgCl from silver nitrate AgNO₃ and sodium chloride NaCl, actual yield must be AgCl mass, not leftover nitrate. Units must match (grams with grams). Yields above 100% usually indicate wet product, unremoved solvent, or weighing the wrong substance.

Manufacturers track yield to optimize cost and waste. The contact process for sulfuric acid H₂SO₄ aims for high conversion of SO₂ to SO₃; catalysts and equilibrium conditions affect yield. Percent yield connects laboratory practice to industrial efficiency — a 95% yield on a multi-tonne batch of sodium hydroxide NaOH still represents substantial lost material. Document molar masses and limiting reactant clearly in every yield report.

Aspirin is synthesized by reacting salicylic acid (C₇H₆O₃, molar mass 138.12 g/mol) with excess acetic anhydride, producing acetylsalicylic acid — aspirin (C₉H₈O₄, molar mass 180.16 g/mol) — in a 1:1 mole ratio. If a student starts with 2.00 g of salicylic acid (the limiting reagent, since acetic anhydride is used in excess), moles of salicylic acid = 2.00 ÷ 138.12 = 0.01448 mol. Since the ratio is 1:1, theoretical moles of aspirin = 0.01448 mol, and theoretical mass = 0.01448 × 180.16 = 2.61 g.

If the student isolates and weighs 2.15 g of purified, dried aspirin crystals after the full recrystallization and filtration procedure, percent yield = (2.15 ÷ 2.61) × 100% = 82.4% — a solid, believable result for an undergraduate synthesis lab, reflecting realistic losses during recrystallization and filtration rather than a flawed calculation.

Even a textbook-perfect reaction with no competing side reactions still loses some yield to unavoidable, mundane causes: a small amount of product inevitably remains dissolved in the filtrate rather than crystallizing out; some solid sticks to glassware during transfers between containers; recrystallization purification, while improving purity, always sacrifices some product mass along with the impurities it removes. Chemists specifically report and track percent yield, rather than assuming 100% conversion, precisely because these small, cumulative losses are a normal and expected part of real experimental chemistry, not a sign that something went wrong.