Introduction
Percentage yield is an important topic in GCSE Chemistry and is commonly tested in exams across AQA, Edexcel and OCR. It measures how much product is formed compared to the maximum amount expected. These questions build on reacting masses and moles calculations. If you are unsure about these topics, you should review them before attempting percentage yield questions using our reacting masses and moles calculations guides. This guide explains how to calculate percentage yield step-by-step, with clear examples and common mistakes highlighted.
Summary
• percentage yield = (actual yield / theoretical yield) × 100
Or
• percentage yield = (what you got / what you should get) × 100
• actual yield = amount made in experiment
• theoretical yield = maximum possible amount
• always include correct units
What Is Percentage Yield?
Percentage yield compares the amount of product actually obtained in a reaction to the maximum possible amount predicted by calculations. In real reactions, the yield is often less than 100% due to losses in the experiment.
The percentage yield is the actual amount obtained via a practical method, this means it can differ from person to person due to variations in their experimental technique. This is distinct from atom economy which is solely based on the chemical reaction used and measures how many of the atoms from the reactants end up in the desired product.
Difference between atom economy and percentage yield
| Percentage Yield | Atom Economy |
|---|---|
| Measures actual product made | Measures useful product in equation |
| Affected by practical losses | Based on balanced symbol equation |
| Experimental | Theoretical/design-based |
Learn more about calculating atom economy in our GCSE Atom Economy guide.
Why is the yield less than 100%?
The yield is often less than 100% due to losses in the experiment, for example:
- Losing some material when transferring all the product from one flask to another (think of cooking a cake, there is always some mixture left in the bowl that is not transferred to the cooking tin),
- Incomplete reactions, the reaction may not have been fully finished
- Reversible reactions, this will be explained further in our future reversible reactions guide.
- Side reactions, where there are other reactions that are taking place.
Percentage Yield Formula
percentage yield = (actual yield / theoretical yield) × 100
Or percentage yield = (what you got / what you should get) × 100
Step-by-Step Method for Percentage Yield Calculations
Step 1. Calculate the theoretical yield using reacting masses calculations you may need to determine the limiting reagent first.
Step 2. Identify the actual yield from the question
Step 3. Use the formula to calculate percentage yield
This method should be followed carefully in exams to ensure method marks are awarded.
How This Appears in Exams
• Calculate the percentage yield of a reaction
• Explain why percentage yield is less than 100%
• Compare efficiency of reactions
Example GCSE Chemistry Percentage Yield Calculation
Example: A reaction produces 8 g of product.
The theoretical yield is 10 g.
percentage yield = (8 ÷ 10) × 100 = 80%
Why Percentage Yield Is Important in GCSE Chemistry
Percentage yield is used to measure how efficient a reaction is. It is important in industrial chemistry where maximising yield can reduce costs and waste. Students should be confident with reacting masses and moles calculations before attempting these questions.
Common Mistakes in Percentage Yield Calculations
Students often lose marks due to avoidable errors, including:
– Mixing up actual and theoretical yield
– Not calculating theoretical yield first
– Forgetting to multiply by 100
– Using incorrect units
Exam Tips
Always check whether the values given are actual or theoretical yield before using the formula.
Remember to check your answer, it is impossible to get a yield over 100%, is this happens you have made a mistake in your working
Final Thoughts
Percentage yield is a key concept in GCSE Chemistry. By following a clear method and practising regularly, students can avoid common mistakes and improve their exam performance. Students should also practise related topics such as reacting masses and moles calculations to build a complete understanding. For a detailed overview of all topics, see our full GCSE Chemistry revision guide.