What is the formula for percent yield?

• A. Percent yield = (actual yield / theoretical yield) × 100%.
• B. Percent yield = (theoretical yield / actual yield) × 100%.
• C. Percent yield = (actual yield × theoretical yield) / 100%.
• D. Percent yield = (theoretical yield − actual yield) / theoretical yield × 100%.

Answer: (A)

Percent yield measures how efficient a reaction was by comparing the amount of product you actually obtained to the maximum amount that could be produced from the reactants (theoretical yield). The actual yield is the real amount collected in the lab, while the theoretical yield is what would result if the reaction went perfectly according to stoichiometry with the limiting reagent.

The best way to express this efficiency is to take the ratio of actual to theoretical yield and multiply by 100 to convert it to a percent: percent yield = (actual yield / theoretical yield) × 100%. The ratio makes the result unitless, and the 100% scales it to a percentage.

For example, if you actually obtained 8 g of product but the theoretical yield was 10 g, the percent yield would be (8/10) × 100% = 80%.

In practice, percent yield is often less than or equal to 100% because losses occur during practice (incomplete reactions, side reactions, product losses during isolation and purification, measurement errors). It can occasionally appear above 100% if impurities or hydration aren’t accounted for in the theoretical calculation, but such cases usually indicate a discrepancy in how the yields were measured or calculated.