Theoretical yield is the maximum mass a reaction could give

1. The maximum mass of product assumes every particle of the limiting reactant is converted.
2. It is calculated from the balanced equation, not measured in the laboratory.
3. Whichever reactant runs out first sets that ceiling, and it is the only one the calculation uses.
4. Any reactant in excess is left over and adds nothing to the mass of product.
5. No real reaction reaches this figure, which is why the calculated value is called theoretical.

Actual yield is the mass actually collected

1. The actual yield is the mass of pure, dry product weighed at the end.
2. It is measured on a balance after the product has been separated and dried.
3. Product still wet with solvent weighs too much, so the sample is dried to constant mass.
4. Product left in the apparatus or lost in filtering never reaches the balance.
5. Both yields are masses, so both are quoted in grams.

Percentage yield compares the two

1. The actual yield is expressed as a percentage of the theoretical yield: $$percentage\; yield=\frac{actual\; yield}{theoretical\; yield}\times 100$$
2. Both masses must be in the same unit, so the units cancel and the answer is a pure number.
3. The answer carries no unit, and the $\%$ sign is part of the number rather than a unit.
4. A value above $100\%$ is impossible and points to an impure or damp sample.
5. A high percentage yield means the process wastes little of the starting material.

A worked percentage yield

1. A reaction has a theoretical yield of $8.0g$ and gives $6.0g$ of dry product.
2. Substituting into the expression gives: $$\frac{6.0}{8.0}\times 100=75\%$$
3. Three quarters of the possible product was recovered, so a quarter was lost or never formed.
4. Rearranging finds a missing figure, so $75\%$ of $8.0g$ gives the actual yield.
5. Dividing the actual yield by the percentage and multiplying by $100$ recovers the theoretical yield.

Why the actual yield falls short

1. The reaction may be incomplete, leaving some reactant unreacted when the mixture is worked up.
2. A reversible reaction reaches equilibrium, so it can never convert all of the reactants.
3. Practical losses occur at every transfer, in filtering, and as product left on the glassware.
4. Side reactions use up reactants to make substances other than the one wanted.
5. Purifying the product removes impurities but removes some product along with them.