AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Many chemical reactions take place in solutions. The concentration of a…
Full specification wording
Concentration of solutions: Many chemical reactions take place in solutions. The concentration of a solution can be measured in mass per given volume of solution, eg grams per dm³ (g/dm³).
A solution forms when a solute dissolves in a solvent, and many reactions occur between dissolved reactants, so their concentration matters.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 1 of 3
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A solution forms when a solute dissolves in a solvent, and many reactions occur between dissolved reactants, so their concentration matters. Concentration expresses how much solute is present in a stated volume of solution. It can be measured as mass per given volume, for example grams per cubic decimetre (g/dm³). To find it, divide the mass of solute in grams by the volume of solution in dm³. For example, dissolving 20 g of sodium chloride in water to make 2 dm³ of solution gives 20 g ÷ 2 dm³ = 10 g/dm³. Remember that 1 dm³ equals 1000 cm³, so a volume in cm³ must be divided by 1000 before use. Concentration is not the same as the mass of solid added, because the final solution volume is what counts.
What earns the marks
- States that concentration is the mass of solute dissolved in a given volume of solution, not simply the mass of solid added.
- Uses the relationship concentration = mass of solute ÷ volume of solution, with mass in g and volume in dm³.
- Converts volumes correctly, using 1 dm³ = 1000 cm³, before calculating concentration.
- Calculates a concentration in g/dm³ from supplied mass and volume data, showing the division clearly.
- Interprets a stated concentration, for example 10 g/dm³, as 10 g of solute in every 1 dm³ of solution.
- Recognises that concentration changes if the same mass of solute is dissolved in a different volume of solution.
Common mistakes and how to put them right
Dividing volume by mass instead of mass by volume.
Put it right by writing concentration = mass ÷ volume and checking that a larger mass in the same volume gives a larger concentration.
Using a volume in cm³ directly in the calculation.
Put it right by converting cm³ to dm³ by dividing by 1000 first.
Treating the mass of solute added as the concentration.
Put it right by dividing that mass by the total volume of the solution formed.
Exam tips
- Write the equation concentration = mass ÷ volume before substituting numbers, so the units and rearrangement are clear.
- Convert every volume to dm³ at the start and label the final answer with g/dm³.
- Check whether the question gives the volume of solution or the volume of solvent, and use the solution volume in the calculation.
Try the lesson
The start of the Concentration of solutions: Many chemical reactions take place in solutions. The concentration of a solution can be measured in mass per given volume of solution, eg grams per dm³ (g/dm³). lesson, made from checked teaching notes for this specification point.
Have a go first
A solution contains 20 g of sodium chloride dissolved in 2 dm³ of solution. What is the concentration of this solution in g/dm³?
Listen
A solution forms when a solute dissolves in a solvent, and many reactions occur between dissolved reactants, so their concentration matters. Concentration expresses how much solute is present in a stated volume of solution. It can be measured as mass per given volume, for example grams per cubic decimetre (g/dm³). To find it, divide the mass of solute in grams by the volume of solution in dm³. For example, dissolving 20 g of sodium chloride in water to make 2 dm³ of solution gives 20 g ÷ 2 dm³ = 10 g/dm³. Remember that 1 dm³ equals 1000 cm³, so a volume in cm³ must be divided by 1000 before use. Concentration is not the same as the mass of solid added, because the final solution volume is what counts. A strong answer earns marks for these points. States that concentration is the mass of solute dissolved in a given volume of solution, not simply the mass of solid added. Uses the relationship concentration = mass of solute ÷ volume of solution, with mass in g and volume in dm³. Converts volumes correctly, using 1 dm³ = 1000 cm³, before calculating concentration. Calculates a concentration in g/dm³ from supplied mass and volume data, showing the division clearly. A common slip: Dividing volume by mass instead of mass by volume. Instead, writing concentration = mass ÷ volume and checking that a larger mass in the same volume gives a larger concentration. Exam tip: Write the equation concentration = mass ÷ volume before substituting numbers, so the units and rearrangement are clear.
See it
Key words
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Concentration
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Unit conversion between cm³ and dm³
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See every definition
- Concentration
- The mass of solute dissolved in a given volume of solution, measured for example in grams per dm³.
- Unit conversion between cm³ and dm³
- A volume in cm³ must be divided by 1000 before use, because 1 dm³ equals 1000 cm³.
Label the diagram
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