What mistakes do students make with Atom economy?

Using the volume of water added instead of the total solution volume. Put it right by making the solution up to the final volume in a volumetric flask before calculating. Forgetting to convert cm³ to dm³. Put it right by dividing the volume in cm³ by 1000 before using the concentration equation. Writing the unit as mol/dm² or mol dm³. Put it right by using mol/dm³, which means moles per cubic decimetre.

What are the exam tips for Atom economy in Chemistry AQA GCSE?

Write the equation concentration = moles ÷ volume in dm³ at the start of a calculation so the rearrangement is clear. Check the unit of every volume and convert to dm³ before substituting values. Give the unit mol/dm³ with your final answer, and round only at the end of the calculation.

AQA · GCSE · Chemistry

Course version: 8462 · Version from Sept 2018

Atom economy: The concentration of a solution can be measured in mol/dm .

Note: This content officially belongs to section 4.3.4 Using concentrations of solutions in mol/dm³ and is strictly Higher Tier only.

Checked against the AQA 8462 specification (opens in a new tab) on 21 September 2026.

Point 4 of 7

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

State that concentration in mol/dm³ is the amount of solute in moles dissolved per cubic decimetre of solution.
Use the relationship concentration = amount in moles ÷ volume in dm³, and rearrange it correctly when needed.
Convert volumes between cm³ and dm³ by dividing or multiplying by 1000 before substituting.
Recognise that the volume used is the total volume of the solution, not the volume of solvent added.
Interpret a stated concentration such as 0.100 mol/dm³ as 0.100 mol of solute in every 1 dm³ of solution.

Common mistakes and how to put them right

Using the volume of water added instead of the total solution volume.
Put it right by making the solution up to the final volume in a volumetric flask before calculating.
Forgetting to convert cm³ to dm³.
Put it right by dividing the volume in cm³ by 1000 before using the concentration equation.
Writing the unit as mol/dm² or mol dm³.
Put it right by using mol/dm³, which means moles per cubic decimetre.

Exam tips

Write the equation concentration = moles ÷ volume in dm³ at the start of a calculation so the rearrangement is clear.
Check the unit of every volume and convert to dm³ before substituting values.
Give the unit mol/dm³ with your final answer, and round only at the end of the calculation.

Try the lesson

The start of the Atom economy: The concentration of a solution can be measured in mol/dm . lesson, made from checked teaching notes for this specification point.

Have a go first

Which of the following units is used to measure the concentration of a solution in terms of moles?

Tip: press 1 to 4 to choose

Listen

Note: This content officially belongs to section 4.3.4 Using concentrations of solutions in mol/dm³ and is strictly Higher Tier only. Concentration indicates how much solute is dissolved in a given volume. Measuring it in mol/dm³ is useful as chemical reactions depend on particle numbers. For example, dissolving 0.5 mol of sodium hydroxide and making the total volume up to 2 dm³ gives a concentration of 0.5 ÷ 2 = 0.25 mol/dm³. The volume used must be the total solution volume, not just the water added. A 1 mol/dm³ solution contains 1 mol of solute per 1 dm³ of solution. Since 1 dm³ = 1000 cm³, volumes in cm³ must be divided by 1000 before calculating. Always read units carefully. A strong answer earns marks for these points. State that concentration in mol/dm³ is the amount of solute in moles dissolved per cubic decimetre of solution. Use the relationship concentration = amount in moles ÷ volume in dm³, and rearrange it correctly when needed. Convert volumes between cm³ and dm³ by dividing or multiplying by 1000 before substituting. Recognise that the volume used is the total volume of the solution, not the volume of solvent added. A common slip: Using the volume of water added instead of the total solution volume. Instead, making the solution up to the final volume in a volumetric flask before calculating. Exam tip: Write the equation concentration = moles ÷ volume in dm³ at the start of a calculation so the rearrangement is clear.

See it

A flowchart with two starting points. The first node is 'Amount of solute in moles'. The second node is 'Volume of solution', which leads to a node 'Volume in cm³? Divide by 1000'. Both the amount in moles and the converted volume lead into a node reading 'Concentration = amount in moles ÷ volume in dm³'. This final calculation leads to an end node 'Concentration in mol/dm³'. — Shows how the amount of solute in moles and the total volume of solution (converted to dm³ if given in cm³) combine to give concentration in mol/dm³.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 2 boxes numbered instead of named. A flowchart with two starting points. The first node is 'Amount of solute in moles'. The second node is 'Volume of solution', which leads to a node 'Volume in cm³? Divide by 1000'. Both the amount in moles and the converted volume lead into a node reading 'Concentration = amount in moles ÷ volume in dm³'. This final calculation leads to an end node 'Concentration in mol/dm³'.

1Box 1
2Box 2

0 of 2 labelled

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