Percentage yield is one of the most important calculation topics in secondary school Chemistry. It appears regularly in WAEC, NECO, and JAMB examinations, and many students lose easy marks because they don’t fully understand it.
In this blog post, you’ll learn:
* What percentage yield means
* The formula for calculating percentage yield
* Step-by-step methods
* Worked examples
* Common exam mistakes to avoid
Let’s break it down in a very simple way.
What Is Percentage Yield in Chemistry?
In Chemistry, when a reaction takes place, we usually expect a certain amount of product to be formed based on calculations. However, in real experiments, the amount of product we actually get is often less than expected.
Percentage yield tells us how efficient a chemical reaction is by comparing:
* What is actually obtained, and
* What we theoretically expected to obtain
In other words,percentage yield shows how successful a chemical reaction is.
Important Terms You Must Understand
Before calculating percentage yield, you need to understand these two key terms:
1. Theoretical Yield
This is the maximum amount of product that should be formed if the reaction goes perfectly.
It is calculated from a balanced chemical equation using stoichiometry.
2. Actual Yield
This is the amount of product actually obtained during the experiment.
Note:
Actual yield is usually less than theoretical yield.
Formula for Calculating Percentage Yield
The formula is:
Percentage Yield = {Actual Yield}divide by{Theoretical Yield} X 100
This formula is very important and should be memorized for exams.
Steps to Calculate Percentage Yield
Whenever you see a percentage yield question, follow these steps:
1. Write the balanced chemical equation (if required)
2. Identify the theoretical yield
3. Identify the actual yield
4. Divide actual yield by theoretical yield
5. Multiply your answer by 100
6. Add the percentage sign (%)
Worked Examples
Example 1
A reaction is expected to produce 20 g of a substance, but only 15 g is obtained.
Calculate the percentage yield.
Solution:
Actual yield = 15 g
Theoretical yield = 20 g
{Percentage Yield} = {15} divide by{20} x 100 = 75%
Answer: 75%
Example 2
If the theoretical yield is 50 g and the actual yield is 40 g, calculate the percentage yield.
Solution:
{40} divide by{50} x 100 = 80%
Answer: 80%
Why Is Percentage Yield Usually Less Than 100%?
In most chemical reactions, percentage yield is not 100%. This is because of:
1. Incomplete reactions
2. Loss of product during filtration or transfer
3. Side reactions producing unwanted substances
4. Impurities in reactants
5. Experimental and measurement errors
These reasons are commonly asked in theory questions.
Common Exam Mistakes to Avoid
Many students lose marks due to simple errors. Avoid these mistakes:
* Forgetting to multiply by 100
* Mixing up actual yield and theoretical yield
* Not writing the formula
* Forgetting the percentage sign (%)
* Skipping calculation steps
Exam Tip:
Always write the formula first — even if you make a mistake later, you can still earn marks.
Why Percentage Yield Is Important
Percentage yield helps chemists to:
* Measure the efficiency of reactions
* Improve industrial chemical processes
* Reduce waste and cost
* Compare different reaction methods
That’s why it is an important topic in both school and industry.
Summary
Percentage yield is a simple but very important Chemistry topic. Once you understand the formula and practice a few examples, it becomes very easy to score full marks in exams.
Key points to remember:
* Percentage yield compares actual yield with theoretical yield
* Formula: Actual Yield ÷ Theoretical Yield × 100
* Always show your working in exams
Further Study
Practice Questions
1. A reaction produces 36 g of a substance instead of the expected 48 g. Calculate the percentage yield.
2. State three reasons why percentage yield is less than 100%.
EXAM-STYLE QUESTIONS AND ANSWERS
Theory Questions
Calculation Questions
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