1.2.8Building Blocks for Understanding
Concentrations Of Solutions — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions
5 min read 9 parts Lesson 14 of 14
Part 1 of 7What concentration means
Solute, solvent, solution
- Solute, solvent, solution A solute dissolves in a solvent, such as water, to form a solution.
- The volume that matters in a concentration calculation is the volume of the whole solution.
Concentration
- Concentration The mass of solute in a given volume of solution;
- a more concentrated solution holds more solute in the same volume.
- A dilute solution holds only a little solute in the same volume.
Units
- Units Concentration is commonly measured in g/dm3, where 1 dm3 = 1000 cm3.
- Higher-tier questions may use mol/dm3, which counts the amount of solute in moles instead of grams.
Part 2 of 7Equation and rearrangements
| To find | Equation | Units |
|---|---|---|
| Concentration | concentration = mass ÷ volume | g/dm3 |
| Mass of solute | mass = concentration × volume | g |
| Volume | volume = mass ÷ concentration | dm3 |
Part 3 of 7Calculating step by step
1
Convert the volume to dm3 by dividing cm3 by 1000.
2
Check the mass is in grams, converting from kilograms or milligrams if needed.
3
Choose the form of the equation that has the unknown on its own.
4
Substitute the values and calculate.
5
Example: 5.0 g dissolved in 250 cm3 is 5.0 ÷ 0.25 = 20 g/dm3.
6
State the answer with its unit.
Part 4 of 7Worked examples
| Question | Working | Answer |
|---|---|---|
| Concentration of 12 g in 500 cm3 | 500 cm3 = 0.5 dm3; 12 ÷ 0.5 | 24 g/dm3 |
| Mass in 200 cm3 of a 15 g/dm3 solution | 15 × 0.200 | 3.0 g |
| Volume holding 9 g at 30 g/dm3 | 9 ÷ 30 = 0.30 dm3 | 300 cm3 |
| Concentration of 0.50 g in 25 cm3 | 0.50 ÷ 0.025 | 20 g/dm3 |
Part 5 of 7Changing a concentration
Same solution, different strength
Dilute by adding water
- Dilute by adding water The mass of solute stays the same but the volume increases, so the concentration falls.
Evaporate solvent
- Evaporate solvent Less volume holds the same solute, so concentration rises.
Add more solute
- Add more solute The mass of solute increases, until the solution is saturated.
- Beyond that point the extra solid simply stays undissolved at the bottom of the container.
Comparing two solutions
- Comparing two solutions Always compare concentrations in the same units;
- a solution with 30 g/dm3 is stronger than one with 12 g/dm3, whatever the volume of each.
Part 6 of 7Where concentration is used
Titrations
- Titrations The concentration of an unknown acid or alkali is found by reacting it with a solution of known concentration.
Rates of reaction
- Rates of reaction A higher concentration means more particles in the same volume, so more frequent collisions and a faster reaction.
Medicine and cleaning
- Medicine and cleaning Doses and cleaning products are made up to a stated concentration so they are safe and effective.
Part 7 of 7Exam technique for concentration questions
1
Underline the units given for volume and mass at the start of the question.
2
Write the conversion line, such as 250 cm3 ÷ 1000 = 0.250 dm3, so marks are available even if the later arithmetic is wrong.
3
Write the equation in words before substituting numbers.
4
Give the final answer with its unit, and round to the same number of significant figures as the data.
Key terms
| Term | Meaning |
|---|---|
| Solute | The substance that dissolves. |
| Solvent | The liquid in which a solute dissolves. |
| Concentration | The mass of solute per unit volume of solution. |
| Saturated solution | A solution that cannot dissolve any more solute at that temperature. |
| Dilute | Containing a small amount of solute in a given volume of solution. |
| dm3 | A cubic decimetre, equal to one litre or 1000 cm3. |
Avoid these mistakes
1
Dividing by cm3 instead of converting to dm3 first.
2
Forgetting that diluting changes the volume, not the mass of solute.
3
Using the volume of solvent instead of the volume of solution.
4
Rearranging the equation incorrectly, for example multiplying when dividing is needed.
5
Dividing by 100 instead of 1000 when changing cm3 to dm3.
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