How To Calculate Percent Yield

Percent yield is a ratio, so it has no units. It is reported as a percentage by multiplying the ratio by one hundred, which is why 162 over 180 becomes 90 percent.

The theoretical yield comes from the balanced equation and the limiting reagent. Convert the mass of each reactant to moles, find which runs out first, and use the mole ratio to get the moles of product.

Real yields are almost always below one hundred percent because product is lost at every transfer, filtration and drying step. A hundred percent yield in a report usually means the product was not dried.

Purity is a separate correction. If the crude product is only ninety five percent pure, the useful mass is ninety five percent of what you weighed, and the yield falls accordingly.

Side reactions are the other main loss. When the starting material can follow two pathways, some of it is diverted into the product you did not want, and that fraction never appears in the yield.

Incomplete reactions leave reactant behind. If the equilibrium does not favour the products, no amount of patience will push the yield to the theoretical value.

Industrial processes aim for high yields but also for high atom economy. A reaction with mediocre yield but almost no waste can be greener than a clean reaction that produces kilograms of byproduct.

The theoretical yield assumes perfect stoichiometry and complete conversion. Errors in the limiting reagent calculation are the most common reason a yield comes out above one hundred percent.

Worked up yields above one hundred percent almost always mean the product contains residual solvent or water, so the measured mass is inflated.

Percent yield is reported alongside percent purity for exactly this reason. One tells you how much of the possible product you made, the other tells you how much of what you made is actually the product.

Percent yield is often reported per step rather than for the whole synthesis. A five step route at ninety percent each gives only 59 percent overall, which is why long routes are designed around the weakest step rather than the strongest.

Catalysts change the rate, not the equilibrium. A beautifully catalysed reaction still stops at the same equilibrium position, so the ceiling on the yield is set by thermodynamics rather than by the catalyst.

Yield is not the only figure of merit. A route that makes two products together with a clean separation may beat a higher yielding route that needs an expensive chromatography step or a hazardous reagent.

The theoretical yield calculation assumes the reaction goes exactly as written. Any competing hydrolysis, oxidation or polymerisation of the intermediate silently removes starting material from the main pathway.