Introduction
Concentration calculations are an important part of GCSE Chemistry and are commonly tested in exams across AQA, Edexcel and OCR. Students are required to calculate concentration in mol/dm³ using mass, moles, and volume. These questions build on knowledge of moles and reacting masses. If you are unsure how to calculate moles, you should read our GCSE Chemistry moles calculations guide first. Relative formula mass (Mr) is also required for these calculations. This guide explains how to calculate concentration step-by-step, with examples and common mistakes highlighted. For a full overview of all GCSE Chemistry topics and how to structure your revision, see our GCSE Chemistry revision overview.
Summary
• concentration = moles ÷ volume (dm³)
• convert volume to dm³ if needed
• use moles = mass ÷ Mr
• rearrange the formula if required
What Is Concentration?
Concentration measures how much of a substance is dissolved in a given volume of solution, the more substance dissolved in the same volume the higher the concentration. When you make up orange squash, the more water you add the more dilute the drink, the orange is in a lower concentration. In GCSE Chemistry, concentration is usually expressed in mol/dm³.
Concentration Formula
concentration = moles ÷ volume (dm³)
Step-by-Step Method for Concentration Calculations
Step 1. Calculate moles, for example using moles = mass ÷ Mr
Step 2. Convert volume into dm³ if necessary (divide cm³ by 1000)
Step 3. Use concentration = moles ÷ volume
This method should be followed carefully in exams to ensure method marks are awarded.
How This Appears in Exams
- Calculate the concentration of a solution
- Calculate the amount of solute in a given volume
- Rearrange the concentration formula
- Use concentrations in titration calculation questions (triple award)
Example GCSE Chemistry Concentration Calculation
Example:
Calculate the concentration of a solution containing 5.85 g of sodium chloride (NaCl) in 500 cm³ of solution.
Step 1:
Calculate moles
Mr of NaCl = 58.5
moles = 5.85 ÷ 58.5 = 0.1 mol
Step 2:
Convert volume 500 cm³ = 0.5 dm³
Step 3:
Calculate concentration
concentration = 0.1 ÷ 0.5 = 0.2 mol/dm³
Why Concentration Is Important in GCSE Chemistry
Concentration is used in many areas of chemistry, including titrations and reaction rates. Students must be confident with moles and relative formula mass (Mr) before attempting these questions. It also links to reacting masses and percentage yield calculations.
Common Mistakes in Concentration Calculations
Students often lose marks due to avoidable errors, including: –
Not converting cm³ to dm³ – this is especially an issue in titration calculations as you have to make this conversion twice and thus not doing it will potentially lose 2 marks.
Using incorrect Mr values
Forgetting to calculate moles first
Rearranging the formula incorrectly
Exam Tip
Always check your units. Volume must be in dm³ when using the concentration formula.
Final Thoughts
Concentration calculations are a key skill in GCSE Chemistry. By following a clear method and practising regularly, students can avoid common mistakes and improve confidence. Students should also practise related topics such as moles, reacting masses, and percentage yield (coming soon) to build a complete understanding. For a complete overview of all topics, see our guide on how to revise GCSE Chemistry effectively.