Mastering concentration calculations is a core requirement for GCSE Chemistry with AQA, whether you are working with masses, volumes, or moles. This guide explains the essential methods you need for exams and practical work.
You will meet different units such as g/dm3 and mol/dm3, and you will need to move between them confidently when solving problems about reacting solutions.
| Key Quantity | Unit | Formula | Typical Use in AQA GCSE |
|---|---|---|---|
| Concentration (mass) | g/dm3 | Mass of solute (g) ÷ Volume of solution (dm3) | Describing how much solid dissolves in water |
| Concentration (moles) | mol/dm3 | Moles of solute ÷ Volume of solution (dm3) | Calculating reacting quantities in titrations |
| Moles | mol | Mass (g) ÷ Mr | Linking mass and number of particles |
| Number of particles | — | Moles × 6 × 1023 | Connecting moles to atoms, molecules, ions |
Concentration Mass g per dm3 Calculations
What This Concentration Means
Concentration in g/dm3 tells you how many grams of a solute are dissolved in each cubic decimetre of solution. A higher number means a more concentrated solution.
How to Calculate and Convert Units
To find concentration in g/dm3, divide the mass of solute in grams by the volume of the solution in dm3. Remember to convert cm3 to dm3 by dividing by 1000 before you calculate.
Concentration Moles per dm3 and Moles
Linking Mass and Moles
In many chemical calculations you work in moles, since reactions happen between set numbers of particles. Use moles to compare amounts of different substances easily.
Using the Mole Formula in Concentration Work
First convert mass to moles by dividing by the relative formula mass, Mr. Then divide moles by volume in dm3 to find concentration in mol/dm3. This is the standard unit for concentration in equations.
Titration Calculations Aqa
What Happens in a Titration
A titration lets you find the concentration of an unknown solution by reacting it with a solution of known concentration. You carefully measure volumes and use them in concentration calculations.
Step by Step Approach
Convert the volume of one reactant to moles using mol/dm3 and volume in dm3. Use the balanced equation to find the mole ratio, then calculate the moles of the other reactant and finally work back to concentration or mass as required.
Dilution and Making Standard Solutions
Why Dilution Matters
When you dilute a solution, the number of moles of solute stays the same, but the volume increases and concentration falls. This lets you prepare less concentrated solutions from stronger ones.
Using the Concentration Equation for Dilution
Apply c1 × v1 = c2 × v2 carefully, making sure all volumes are in the same units and that concentrations are in the correct units. Check that your answer is smaller than the original concentration when you dilute.
Key Points for AQA GCSE Chemistry
- Distinguish between concentration in g/dm3 and mol/dm3 and choose the correct unit for the question.
- Convert volumes from cm3 to dm3 by dividing by 1000 before calculating concentration.
- Use moles and balanced equations to link substances in titration and dilution problems.
- Check that your calculations make sense, for example concentration should drop when you dilute a solution.
FAQ
Reader questions
How do I calculate concentration in mol/dm3 from a titration given volumes in cm3?
Convert the volume in cm3 to dm3 by dividing by 1000, then multiply by the concentration in mol/dm3 of the known solution to find moles. Use the balanced equation to find moles of the unknown, and divide by the unknown volume in dm3 to find its concentration.
How can I find the mass of solute needed to make a solution of a specific concentration in g/dm3?
Multiply the desired concentration in g/dm3 by the volume in dm3 of the solution you want to prepare to find the required mass of solute.
What should I do if the volume is given in cm3 when calculating concentration in mol/dm3?
Always convert cm3 to dm3 by dividing by 1000 before using it in the concentration formula, or adjust the calculation so the units match.
How do I use the equation c1v1 = c2v2 correctly during dilution?
Use consistent units for volume and concentration, keep the same units on both sides of the equation, and remember that the product c × v before dilution equals the product after dilution.