Percentage Yield

The theoretical yield is the maximum mass of product that could form, calculated from the mass of the limiting reactant and the balanced equation. The actual yield is the mass you really obtain.percentage yield = (actual yield ÷ theoretical yield) × 100Worked example: 5.00 g of calcium carbonate is heated and 2.40 g of calcium oxide is collected. Calculate the percentage yield.CaCO₃(s) → CaO(s) + CO₂(g)amount of CaCO₃ = 5.00 ÷ 100.1 = 0.04995 mol, so at most 0.04995 mol of CaO can formtheoretical yield of CaO = 0.04995 × 56.1 = 2.80 gpercentage yield = (2.40 ÷ 2.80) × 100 = 85.7%Yields are almost always below 100% because:• the reaction may be incomplete, or reversible and reach equilibrium• side reactions may form other products• product is lost during transfer, filtration or purification (for example recrystallisation)• the reactants may be impureA yield above 100% points to a problem with the product, most often that it is still wet or contains impurities.Percentage yield tells you how efficient the practical process was. It does not tell you how much waste the reaction itself makes; that is measured by atom economy.