Percentage Yield and Atom Economy

This lesson covers percentage yield and atom economy as required by the AQA GCSE Combined Science Trilogy specification (8464). You will learn how to calculate each, understand why they matter, and know the differences between them.

Percentage Yield

What Is Percentage Yield?

The percentage yield compares the amount of product you actually get in an experiment with the amount you theoretically could get (from the calculation).

$\text{percentage yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100\%$

Why Is Percentage Yield Always Less Than 100%?

In practice, you almost never get 100% yield. The reasons include:

Reason	Explanation
Incomplete reaction	The reaction may not go to completion — some reactant remains unreacted
Side reactions	Reactants may form unwanted by-products instead of the desired product
Loss during transfer	Product is lost when transferring between containers, filtering, or washing
Loss during purification	Some product may be lost during evaporation, crystallisation or drying
Reversible reactions	If the reaction is reversible, it reaches equilibrium and does not go to completion

Exam Tip (AQA 8464): The percentage yield can never exceed 100%. If your calculation gives more than 100%, you have made an error.

Worked Examples — Percentage Yield

Example 1

Question: The theoretical yield of copper sulfate from a reaction is 20 g, but a student obtains only 15 g. What is the percentage yield?

$\text{percentage yield} = \frac{15}{20} \times 100\% = 75\%$

Example 2

Question: A reaction has a percentage yield of 80% and a theoretical yield of 50 g. What is the actual yield?

$\text{actual yield} = \frac{80}{100} \times 50 = 40 \text{ g}$

Example 3

Question: A student heats 10 g of calcium carbonate. The theoretical yield of calcium oxide is 5.6 g, but the student obtains 4.2 g. What is the percentage yield?

$\text{percentage yield} = \frac{4.2}{5.6} \times 100\% = 75\%$

Atom Economy

What Is Atom Economy?

Atom economy measures how much of the total mass of reactants ends up as the desired product (rather than waste by-products). It is a theoretical calculation based on the balanced equation.

$\text{atom economy} = \frac{M_r \text{ of desired product}}{\text{sum of } M_r \text{ of all products}} \times 100\%$

Why Does Atom Economy Matter?

High Atom Economy	Low Atom Economy
Less waste produced	More waste (by-products) produced
More sustainable	Less sustainable
Lower disposal costs	Higher disposal costs
Better for the environment	Greater environmental impact

Exam Tip (AQA 8464): A reaction with only one product always has 100% atom economy (e.g. addition reactions, synthesis reactions with no by-products).

Worked Examples — Atom Economy

Example 1 — Thermal Decomposition

$\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$

If CaO is the desired product:

$M_r$ of CaO = 56, $M_r$ of $\text{CO}_2$ = 44

$\text{atom economy} = \frac{56}{56 + 44} \times 100\% = \frac{56}{100} \times 100\% = 56\%$

Example 2 — Hydrogen from Zinc

$\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$

If $\text{H}_2$ is the desired product:

$M_r$ of $\text{ZnCl}_2$ = 136, $M_r$ of $\text{H}_2$ = 2

$\text{atom economy} = \frac{2}{136 + 2} \times 100\% = \frac{2}{138} \times 100\% = 1.4\%$

Very low atom economy — most of the mass ends up as the by-product $\text{ZnCl}_2$ .

Example 3 — 100% Atom Economy

$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$

There is only one product (MgO), so:

$\text{atom economy} = \frac{M_r \text{ of MgO}}{M_r \text{ of MgO}} \times 100\% = 100\%$

Comparing Percentage Yield and Atom Economy

flowchart LR
    subgraph YD["Percentage Yield"]
        Y1["Actual yield vs\ntheoretical yield"]
        Y2["Depends on\nexperimental technique"]
        Y3["Can be improved\nby better technique"]
    end
    subgraph AE["Atom Economy"]
        A1["Desired product mass vs\ntotal product mass"]
        A2["Depends on\nthe reaction itself"]
        A3["Can only be changed\nby using a different\nreaction pathway"]
    end
    style YD fill:#3b82f6,color:#fff,stroke:#2563eb
    style AE fill:#22c55e,color:#fff,stroke:#16a34a

Feature	Percentage Yield	Atom Economy
What it measures	How much product you actually get	How much of the reactant mass becomes the desired product
Depends on	Experimental technique and conditions	The balanced equation and which product you want
Can be improved by	Better technique, purer reagents, optimised conditions	Choosing a different reaction pathway
Maximum value	100% (rarely achieved)	100% (only if there is one product)
Type	Experimental (practical)	Theoretical (calculated)

Why Do Companies Care About Both?

Chemical companies want both high percentage yield AND high atom economy:

High percentage yield → more product from each batch → more profit.
High atom economy → less waste → lower disposal costs and less environmental impact.

A reaction with 100% atom economy but only 10% yield is wasteful in practice. A reaction with 100% yield but 5% atom economy produces mountains of waste.

Common Mistakes to Avoid

Mistake	Correction
Confusing yield and atom economy	Yield is experimental; atom economy is theoretical
Using the $M_r$ of reactants in atom economy	Atom economy uses the products — desired product ÷ all products
Thinking atom economy can be improved by better technique	Only a different reaction pathway changes atom economy
Calculating yield > 100%	This indicates an error — check your working

Percentage Yield and Atom Economy

Percentage Yield and Atom Economy