Concentration of solutions — AQA GCSE Combined Science
Test yourself on Concentration of solutions with AQA GCSE practice questions.
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Concentration of solutions explained
A solution forms when a solute dissolves in a solvent, and many reactions occur between dissolved reactants, so their concentration matters.
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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.
calculate the mass of solute in a given volume of solution of known concentration in terms of mass per given volume of solution
Concentration tells you how much solute is dissolved in each unit volume of solution, commonly expressed in g/dm³. To find the mass of solute, rearrange the relationship concentration = mass ÷ volume so that mass = concentration × volume. For example, a 250 cm³ solution of concentration 40 g/dm³ contains 40 × 0.250 = 10 g of solute. The crucial step is unit conversion: 1 dm³ = 1000 cm³, so divide a volume in cm³ by 1000 before multiplying. Check the answer by reasoning: a larger volume at the same concentration must contain proportionally more solute, and a more concentrated solution in the same volume must contain more solute. Always attach the correct unit, usually grams, and consider whether the value is sensible for the solute involved.
(HT only) explain how the mass of a solute and the volume of a solution is related to the concentration of the solution.
Concentration measures how much solute is dissolved in a given volume of solution, linking mass and volume through concentration = mass ÷ volume. As Higher Tier content, students must explain these relationships. For a fixed volume, increasing the mass of solute raises the concentration proportionally: doubling the mass doubles the concentration. For a fixed mass of solute, increasing the volume lowers the concentration: dissolving the same mass in twice the volume halves the concentration. This explains why a small volume of a concentrated solution can contain the same solute mass as a larger volume of a dilute solution. Expressing the relationship as an equation allows predictions about how concentration changes when variables are altered.
Your focus
- Define concentration as the mass of solute per given volume of solution.
- Convert a volume in cm³ to dm³ correctly.
- Calculate concentration in g/dm³ from supplied mass and volume values.
Show all 9 objectives
- Select and rearrange the concentration relationship to make mass the subject.
- Convert volumes between cm³ and dm³ correctly before calculation.
- Calculate the mass of solute in a solution of known concentration and volume, giving the answer with an appropriate unit.
- Describe how concentration depends on both the mass of solute and the volume of solution.
- Explain the direct and inverse proportionalities using the concentration equation.
- Apply proportional reasoning to predict how concentration changes when mass or volume is altered.
Concentration of solutions exam tips
Marking 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.
- 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.
- State the relationship concentration = mass of solute ÷ volume of solution and rearrange it correctly to mass = concentration × volume.
- Convert a volume given in cm³ into dm³ by dividing by 1000 before substituting, since concentration is normally quoted in g/dm³.
- Substitute the known concentration and converted volume into mass = concentration × volume and evaluate accurately.
- Give the final mass with the correct unit, typically g, and check that the magnitude is reasonable for the volume and concentration involved.
- Interpret the result in context, for example stating the mass of solute present in the stated volume of that particular solution.
- State that concentration is directly proportional to the mass of solute when the volume of solution is kept constant.
- State that concentration is inversely proportional to the volume of solution when the mass of solute is kept constant.
- Express the relationship as concentration = mass of solute ÷ volume of solution and use it to justify the proportionalities.
- Apply the relationship to a numerical or practical example, such as showing that dissolving 20 g in 2 dm³ gives 10 g/dm³ whereas the same 20 g in 1 dm³ gives 20 g/dm³.
Examiner 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.
- 💡Write down the equation mass = concentration × volume before substituting any numbers, so the examiner can follow your method.
- 💡Convert every volume to dm³ in a clearly shown step, since unit errors are the most common source of lost marks in this topic.
- 💡Estimate the answer first, for example 40 g/dm³ × 0.25 dm³ is about 10 g, then compare your calculated value with the estimate to catch slips.
- 💡Use the words directly proportional and inversely proportional explicitly, since Higher Tier explanation questions reward precise relationship language.
- 💡Support your explanation with a simple numerical example, as this demonstrates understanding rather than repeating the definition.
Common Mistakes
- Dividing volume by mass instead of mass by volume; correct this 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; correct this by converting cm³ to dm³ by dividing by 1000 first.
- Treating the mass of solute added as the concentration; correct this by dividing that mass by the total volume of the solution formed.
- Multiplying concentration by a volume left in cm³ instead of converting to dm³; correct this by dividing cm³ by 1000 first, so 500 cm³ becomes 0.500 dm³.
- Using mass = volume ÷ concentration instead of mass = concentration × volume; correct this by rearranging concentration = mass ÷ volume carefully before substituting.
- Forgetting the unit on the answer or writing an impossible unit such as g/dm³ for a mass; correct this by checking that the unit matches the quantity calculated, which is grams here.
- Claiming that adding more solvent increases concentration; correct this by explaining that adding solvent increases volume and therefore decreases concentration for the same mass of solute.
- Treating mass and volume as interchangeable or ignoring one of them; correct this by always relating concentration to both the mass of solute and the volume of solution together.
- Saying concentration is proportional to volume rather than inversely proportional; correct this by noting that doubling the volume halves the concentration when the mass is unchanged.