A Level Chemistry OCR Practice Exam

How is percentage yield calculated in chemistry?

1. % yield = theoretical amount / actual amount × 100

2. % yield = actual amount × 100 / theoretical amount

3. % yield = actual amount / theoretical amount × 100

4. % yield = theoretical amount × 100 / actual amount

The correct answer is: % yield = actual amount / theoretical amount × 100

Percentage yield is calculated by comparing the actual amount of product obtained from a chemical reaction to the theoretical amount that could be produced based on the stoichiometry of the reaction. The correct formula is that percentage yield equals the actual amount of product divided by the theoretical amount, followed by multiplying by 100 to convert it to a percentage. Using this formula allows chemists to evaluate the efficiency of a reaction. If the actual yield is close to the theoretical yield, it indicates a high percentage yield and suggests that the reaction proceeded well with minimal losses. This is particularly important in practical chemistry, where various factors such as side reactions, incomplete reactions, and losses during product recovery can affect the yield. This understanding highlights why the other proposed formulas do not represent the correct method for calculating percentage yield. They either rearrange the terms incorrectly or misplace the relationship between actual and theoretical yield. Thus, using the ratio of actual to theoretical yield ensures a reliable measure of a reaction's success.