Concentration of solutions
By the end of this chapter you will be able to
- Convert cm3 to dm3 by dividing by 1000
- Calculate the concentration of a solution in g/dm3
- Calculate the mass of solute in a given volume of a solution
- [H] Calculate concentration in mol/dm3, and moles from concentration and volume
- Predict the mass of product from a reaction in solution, and explain what dilution does
You should already know
- Moles, Mr and mass (Chapter 8)
- Balanced equations and mass ratios (Chapter 7)
Where this leads
- Acids, alkalis and titration (Chapter 10)
- Electrolysis of solutions (Chapter 11)
GCSE Chemistry. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Convert volumes from cm3 to dm3 and calculate concentration in g/dm3, Calculate the mass of solute from a concentration and a volume, [H] Calculate concentration in mol/dm3 and the number of moles of solute, Use concentration to predict the mass of product in a reaction in solution, Explain dilution of a solution, and how it changes the concentration.
Exercise P Do you remember?
Method. One litre is 1000 millilitres.
- Recall. 1 litre = 1000 cm3.1000
- Write the answer. 1000
Check. 1000 / 1000 = 1 litre. ✓
Method. Divide 10 by 250.
- Divide. 10 / 250 = 0.04.0.04
- Write the answer. 0.04
Check. 0.04 x 250 = 10. ✓
Method. Multiply, then divide by 100.
- Multiply. 40 x 10 = 400.400
- Divide. 400 / 100 = 4.4
- Write the answer. 4
Check. 4 x 100 = 400. ✓
Method. Add the Ar values.
- Add. 23 + 16 + 1 = 40.40
- Write the answer. 40
Check. Sodium hydroxide has Mr 40. ✓
Method. Multiply.
- Multiply. 5 x 0.5 = 2.5.2.5
- Write the answer. 2.5
Check. 2.5 x 2 = 5. ✓
Concentration in g/dm3
The concentration of a solution is the mass of solute dissolved in each unit volume of solvent (or solution). In GCSE chemistry it is often given in g/dm3. 1 dm3 = 1000 cm3, so to convert from cm3 to dm3, divide by 1000.
| Volume | In dm3 |
|---|---|
| 50 cm3 | 0.05 |
| 250 cm3 | 0.25 |
| 500 cm3 | 0.5 |
| 1000 cm3 | 1 |
Key ideas
- Concentration (g/dm3) = mass of solute (g) / volume of solution (dm3).
- To convert cm3 to dm3, divide by 1000.
- A bigger mass in the same volume gives a higher concentration.
- Always convert the volume to dm3 before you divide.
Method. Convert the volume to dm3, then divide.
- Convert. 250 cm3 = 250 / 1000 = 0.25 dm3.0.25 dm3
- Divide. concentration = 10 / 0.25 = 40.40
- Write the answer. 40
Check. 0.25 x 40 = 10 g. ✓
Method. Convert the volume, then divide the mass by it.
- Convert. 500 cm3 = 0.5 dm3.0.5 dm3
- Divide. concentration = 5 / 0.5 = 10.10
- Write the answer. 10
Check. Twice the volume gives half the concentration of 20 g/dm3. ✓
Method. Convert 50 cm3 first.
- Convert. 50 cm3 = 0.05 dm3.0.05 dm3
- Divide. concentration = 3 / 0.05 = 60.60
- Write the answer. 60
Check. 0.05 x 60 = 3 g. ✓
Watch out
Watch out: the volume must be in dm3 for g/dm3. Dividing by 50 cm3 directly gives the wrong answer by a factor of 1000.
Exam tip
Exam tip. Write “convert the volume” as your first line. Then the division is simple.
Exercise 9A Concentration in g/dm3
Method. Recall the relationship.
- Recall. 1 dm3 = 1000 cm3.1000
- Write the answer. 1000
Check. So divide by 1000 to convert cm3 to dm3. ✓
Method. Divide by 1000.
- Divide. 250 / 1000 = 0.25.0.25
- Write the answer. 0.25
Check. 0.25 x 1000 = 250. ✓
Method. Divide by 1000.
- Divide. 500 / 1000 = 0.5.0.5
- Write the answer. 0.5
Check. 0.5 x 1000 = 500. ✓
Method. Divide by 1000.
- Divide. 50 / 1000 = 0.05.0.05
- Write the answer. 0.05
Check. 0.05 x 1000 = 50. ✓
Method. Multiply by 1000.
- Multiply. 2.5 x 1000 = 2500.2500
- Write the answer. 2500
Check. 2500 / 1000 = 2.5 dm3. ✓
Method. The volume is already 1 dm3.
- Divide. 20 / 1 = 20.20
- Write the answer. 20
Check. The concentration is 20 g/dm3. ✓
Method. Divide by 1.
- Divide. 8 / 1 = 8.8
- Write the answer. 8
Check. Check: 8 x 1 = 8 g. ✓
Method. Convert first.
- Convert. 250 cm3 = 0.25 dm3.0.25
- Divide. 10 / 0.25 = 40.40
- Write the answer. 40
Check. 0.25 x 40 = 10. ✓
Method. Multiply the concentration by the volume.
- Multiply. mass = 15 x 2 = 30 g.30
- Write the answer. 30
Check. 30 / 2 = 15 g/dm3. ✓
Method. Convert, then divide.
- Convert. 300 cm3 = 0.3 dm3.0.3
- Divide. 6 / 0.3 = 20.20
- Write the answer. 20
Check. 0.3 x 20 = 6. ✓
Method. Compare the concentrations.
- 250. 10 g in 0.25 dm3 is 40 g/dm3.40
- 500. 10 g in 0.5 dm3 is 20 g/dm3.20
- Decide. The smaller volume is more concentrated, so 250.250
- Write the answer. 250
Check. The same mass in less solvent is more concentrated. ✓
Method. Look at the unit.
- Unit. The unit g/dm3 uses dm3 as the volume unit.dm3
- Conclude. So the volume must be converted before dividing.convert
- Write the answer. Accept any sentence that says the unit of concentration uses dm3, so the volume must be in dm3 to match.
Check. Dividing by a cm3 volume gives the wrong answer by 1000. ✓
Method. Convert the volume.
- Convert. 250 cm3 = 0.25 dm3.0.25 dm3
- Divide. 25 / 0.25 = 100 g/dm3.100
- Write the answer. Accept any sentence that says 250 cm3 is 0.25 dm3, so the concentration is 100 g/dm3.
Check. The pupil forgot to convert the volume. ✓
Mass of solute from concentration and volume
If the concentration and the volume are known, the mass of solute is:
mass (g) = concentration (g/dm3) x volume (dm3)
The volume must be in dm3. If it is in cm3, convert it first.
| Quantity | Unit | Formula |
|---|---|---|
| concentration | g/dm3 | mass (g) / volume (dm3) |
| concentration [H] | mol/dm3 | moles / volume (dm3) |
| mass of solute | g | concentration x volume (dm3) |
| moles of solute [H] | mol | concentration (mol/dm3) x volume (dm3) |
Key ideas
- Mass of solute = concentration x volume (in dm3).
- Volume of solution = mass of solute / concentration.
- Convert cm3 to dm3 first.
- The answer has the unit of the solute: grams.
Method. Multiply the concentration by the volume.
- Multiply. mass = 25 x 0.4.25 x 0.4
- Answer. mass = 10 g.10
- Write the answer. 10
Check. 10 / 0.4 = 25 g/dm3. ✓
Method. Convert 50 cm3 to dm3 first.
- Convert. 50 cm3 = 0.05 dm3.0.05
- Multiply. mass = 60 x 0.05 = 3.3
- Write the answer. 3
Check. 3 / 0.05 = 60 g/dm3. ✓
Method. Divide the mass by the concentration, then convert to cm3.
- Divide. volume = 5 / 20 = 0.25 dm3.0.25 dm3
- Convert. 0.25 x 1000 = 250 cm3.250
- Write the answer. 250
Check. 250 / 1000 = 0.25 dm3. ✓
Watch out
Watch out: multiply by the volume in dm3. Using cm3 directly multiplies the answer by 1000 too much.
Exam tip
Exam tip. Write the concentration, the volume in dm3, and the calculation on separate lines.
Exercise 9B Mass of solute from concentration and volume
Method. Multiply 30 by 1.
- Multiply. 30 x 1 = 30.30
- Write the answer. 30
Check. The volume is 1 dm3. ✓
Method. Multiply 15 by 2.
- Multiply. 15 x 2 = 30.30
- Write the answer. 30
Check. Double the volume gives double the mass. ✓
Method. Multiply 40 by 0.5.
- Multiply. 40 x 0.5 = 20.20
- Write the answer. 20
Check. Half a litre is half of 40. ✓
Method. Multiply 25 by 0.4.
- Multiply. 25 x 0.4 = 10.10
- Write the answer. 10
Check. 10 / 0.4 = 25. ✓
Method. Convert 250 cm3 to 0.25 dm3, then multiply.
- Convert. 250 cm3 = 0.25 dm3.0.25
- Multiply. 80 x 0.25 = 20.20
- Write the answer. 20
Check. 80 / 4 = 20. ✓
Method. Convert, then multiply.
- Convert. 100 cm3 = 0.1 dm3.0.1
- Multiply. 50 x 0.1 = 5.5
- Write the answer. 5
Check. 5 / 0.1 = 50 g/dm3. ✓
Method. Divide the mass by the concentration.
- Divide. volume = 5 / 20 = 0.25 dm3.0.25
- Write the answer. 0.25
Check. 0.25 x 20 = 5 g. ✓
Method. Divide the mass by the concentration, then convert.
- Divide. volume = 6 / 12 = 0.5 dm3.0.5
- Convert. 0.5 x 1000 = 500 cm3.500
- Write the answer. 500
Check. 500 cm3 = 0.5 dm3. ✓
Method. Multiply the concentration by the volume.
- Multiply. 58.5 x 0.2 = 11.7.11.7
- Write the answer. 11.7
Check. 11.7 / 0.2 = 58.5 g/dm3. ✓
Method. Convert and divide.
- Convert. 500 cm3 = 0.5 dm3.0.5
- Divide. 15 / 0.5 = 30.30
- Write the answer. 30
Check. 0.5 x 30 = 15 g. ✓
Method. Think about what concentration measures.
- Measure. Concentration is the mass of solute in each dm3.mass per volume
- Conclude. A concentrated solution has more solute in the same volume.more solute
- Write the answer. Accept any sentence that says it has a larger mass of solute in the same volume.
Check. Concentration measures solute per unit volume. ✓
Concentration in mol/dm3 [H]
[H] Higher tier only. The concentration in mol/dm3 is the number of moles of solute in each dm3 of solution: concentration (mol/dm3) = moles / volume (dm3). So moles = concentration x volume (dm3). To change g/dm3 into mol/dm3, divide by the Mr.
Key ideas
- mol/dm3 = moles / volume (dm3).
- moles = mol/dm3 x volume (dm3).
- Concentration in g/dm3 / Mr = concentration in mol/dm3.
- Convert cm3 to dm3 first.
Method. Divide the moles by the volume.
- Divide. concentration = 0.5 / 2.0.5 / 2
- Answer. concentration = 0.25 mol/dm3.0.25
- Write the answer. 0.25
Check. 0.25 x 2 = 0.5 mol. ✓
Method. Multiply the concentration by the volume.
- Multiply. moles = 0.1 x 0.25.0.1 x 0.25
- Answer. moles = 0.025.0.025
- Write the answer. 0.025
Check. 0.025 / 0.25 = 0.1 mol/dm3. ✓
Method. Divide the g/dm3 value by the Mr.
- Divide. 40 / 40 = 1.1
- Units. The concentration is 1 mol/dm3.1 mol/dm3
- Write the answer. 1
Check. One mole of NaOH is 40 g. ✓
Watch out
Watch out: mol/dm3 and g/dm3 are different units. Convert between them by dividing or multiplying by the Mr, not by 1000.
Exam tip
Exam tip. Write down the units on every line. If the answer has mol, it must be moles; if g, it must be a mass.
Exercise 9C Concentration in mol/dm3 [H]
Method. Divide 1 by 1.
- Divide. 1 / 1 = 1.1
- Write the answer. 1
Check. A 1 mol/dm3 solution is called a molar solution. ✓
Method. Divide the moles by the volume.
- Divide. 2 / 4 = 0.5.0.5
- Write the answer. 0.5
Check. 0.5 x 4 = 2 mol. ✓
Method. Multiply the concentration by the volume.
- Multiply. 0.5 x 2 = 1.1
- Write the answer. 1
Check. 1 / 2 = 0.5 mol/dm3. ✓
Method. Multiply the concentration by the volume.
- Multiply. 0.1 x 0.25 = 0.025.0.025
- Write the answer. 0.025
Check. 0.025 / 0.25 = 0.1. ✓
Method. Convert first, then multiply.
- Convert. 50 cm3 = 0.05 dm3.0.05
- Multiply. 0.2 x 0.05 = 0.01.0.01
- Write the answer. 0.01
Check. 0.01 / 0.05 = 0.2 mol/dm3. ✓
Method. Convert the volume, then divide.
- Convert. 50 cm3 = 0.05 dm3.0.05
- Divide. 0.01 / 0.05 = 0.2.0.2
- Write the answer. 0.2
Check. 0.2 x 0.05 = 0.01. ✓
Method. Find the moles of NaOH, then divide by the volume.
- Moles. moles = 4.0 / 40 = 0.1 mol.0.1
- Divide. concentration = 0.1 / 0.25 = 0.4 mol/dm3.0.4
- Write the answer. 0.4
Check. 0.4 x 0.25 = 0.1 mol. ✓
Method. Divide by the Mr.
- Divide. 60 / 40 = 1.5.1.5
- Write the answer. 1.5
Check. 1.5 x 40 = 60 g/dm3. ✓
Method. Multiply the concentration by the volume taken.
- Multiply. 0.2 x 0.05 = 0.01.0.01
- Write the answer. 0.01
Check. This matches the 50 cm3 example. ✓
Method. Think about the equation.
- Moles. Balanced equations use moles, so a concentration in mol/dm3 gives the moles directly.moles
- Conclude. This makes it easy to compare reactant amounts.compare
- Write the answer. Accept any sentence that says it gives the number of moles, which relates to the balanced equation.
Check. Moles link solutions to the mass ratios in Chapter 8. ✓
Predicting the mass of product in a reaction in solution
To predict the mass of product from a reaction in solution: (1) find the moles of solute from its concentration and volume; (2) use the ratio from the balanced equation to find the moles of product; (3) change moles to mass (x Mr). The example below uses a solution of copper sulfate with excess magnesium: Mg + CuSO4 -> MgSO4 + Cu.
Key ideas
- The method: concentration and volume give the moles of solute; the balanced equation gives the moles of product; mass = moles x Mr.
- In a reaction with excess of one reactant, all of the other is used up.
- Use cm3 converted to dm3 for the volume.
- Check that the answer is sensible: a bigger volume or concentration gives more product.
Method. Find the moles of CuSO4, then the mass of Cu.
- Moles. moles = 0.5 x 0.05 = 0.025 mol.0.025
- Ratio. CuSO4 : Cu = 1 : 1, so 0.025 mol Cu.0.025 mol
- Mass. mass = 0.025 x 64 = 1.6 g.1.6
- Write the answer. 1.6
Check. Magnesium is in excess, so all the copper sulfate reacts. ✓
Method. Use the same moles of product as the moles of copper sulfate.
- Moles. The equation gives 1 mol MgSO4 for 1 mol CuSO4: 0.025 mol.0.025
- Mass. mass = 0.025 x 120 = 3.0 g.3.0
- Write the answer. 3
Check. Mass conservation is not needed here: 3.0 g MgSO4 forms. ✓
Method. Multiply the concentration by the volume.
- Multiply. mass = 20 x 0.5.20 x 0.5
- Answer. mass = 10 g.10
- Write the answer. 10
Check. Check: 10 / 0.5 = 20 g/dm3. ✓
Watch out
Watch out: check which reactant is in excess. If magnesium is in excess, use the copper sulfate to find the product, not the magnesium.
Exam tip
Exam tip. Write each step on its own line: moles of solute, moles of product, mass of product.
Exercise 9D Reactions in solution
Method. Concentration times volume.
- Multiply. 0.5 x 1 = 0.5 mol.0.5
- Write the answer. 0.5
Check. The volume is already in dm3. ✓
Method. Moles of Cu equal moles of CuSO4; multiply by Ar 64.
- Multiply. 0.025 x 64 = 1.6.1.6
- Write the answer. 1.6
Check. 1.6 / 64 = 0.025 mol. ✓
Method. Multiply moles by Mr.
- Multiply. 0.025 x 120 = 3.3
- Write the answer. 3
Check. 3 / 120 = 0.025 mol. ✓
Method. Read the numbers in front.
- Read. Each has 1 in front, so the ratio is 1 : 1.1 : 1
- Write the answer. 1 : 1
Check. The equation is balanced. ✓
Method. Convert 50 cm3 and multiply.
- Convert. 50 cm3 = 0.05 dm3.0.05
- Multiply. 0.5 x 0.05 = 0.025.0.025
- Write the answer. 0.025
Check. The same as the worked example. ✓
Method. Moles of copper sulfate, then mass of copper.
- Moles. 0.5 x 0.05 = 0.025 mol.0.025
- Mass. 0.025 x 64 = 1.6 g.1.6
- Write the answer. 1.6
Check. Copper has Ar 64. ✓
Method. Convert 250 cm3, then multiply.
- Convert. 250 cm3 = 0.25 dm3.0.25
- Multiply. 40 x 0.25 = 10.10
- Write the answer. 10
Check. 10 / 0.25 = 40 g/dm3. ✓
Method. Divide the moles by the volume in dm3.
- Convert. 50 cm3 = 0.05 dm3.0.05
- Divide. 0.025 / 0.05 = 0.5 mol/dm3.0.5
- Write the answer. 0.5
Check. Matches the copper sulfate example. ✓
Method. Think about the reaction.
- Excess. If magnesium is in excess, all the copper sulfate is used up.all used
- Conclude. So the amount of copper formed depends on the copper sulfate only.copper sulfate only
- Write the answer. Accept any sentence that says it makes sure all the copper sulfate reacts.
Check. This is the idea of a limiting reactant from Chapter 8. ✓
Method. Think about the moles.
- Moles. The moles of solute depend on concentration times volume.x volume
- Conclude. Doubling the volume doubles the moles, so the copper mass doubles.doubles
- Write the answer. Accept any sentence that says the moles of copper sulfate are doubled, so the product is doubled.
Check. The product is proportional to the moles of reactant. ✓
Dilution
Dilution means adding solvent (usually water) to a solution. The mass or moles of solute stay the same, but the volume increases, so the concentration decreases. If the volume is doubled, the concentration is halved.
Key ideas
- Dilution adds solvent; the moles or mass of solute stay the same.
- The concentration decreases when the volume increases.
- Doubling the volume halves the concentration.
- A solution with the same solute in less volume is more concentrated.
Method. The mass of solute stays the same; find it first.
- Mass. mass = 2 x 0.05 = 0.1 g.0.1 g
- New concentration. concentration = 0.1 / 0.1 = 1 g/dm3.1
- Write the answer. 1
Check. The volume doubled, so the concentration halved from 2 to 1. ✓
Method. Moles stay the same; divide by the new volume.
- Moles. The moles are unchanged.the same
- Concentration. The concentration halves: 0.2 / 2 = 0.1 mol/dm3.0.1
- Write the answer. 0.1
Check. Doubling the volume halves the concentration. ✓
Method. The solute mass is the same; divide by 4.
- Factor. The volume is multiplied by 4.4
- Concentration. 40 / 4 = 10 g/dm3.10
- Write the answer. 10
Check. The mass of solute stays at 40 g per dm3 of the original volume. ✓
Watch out
Watch out: dilution lowers the concentration, but it does not change the mass or the moles of solute. Do not divide the mass of solute when you dilute.
Exam tip
Exam tip. Ask: has the amount of solute changed? If not, the new concentration is the old amount divided by the new volume.
Exercise 9E Dilution
Method. Recall what dilution does.
- Recall. Only solvent is added, so the solute is unchanged.no
- Write the answer. no
Check. The volume changes instead. ✓
Method. Recall what dilution does.
- Recall. The same solute is spread through a larger volume, so the concentration falls.decrease
- Write the answer. decrease
Check. Dilution makes a solution weaker. ✓
Method. Halve the concentration when the volume doubles.
- Halve. 20 / 2 = 10.10
- Write the answer. 10
Check. 10 x 2 = 20 g in the same mass of solute. ✓
Method. Divide by 3.
- Divide. 30 / 3 = 10.10
- Write the answer. 10
Check. 10 x 3 = 30. ✓
Method. Find the mass before dilution.
- Before. mass = 2 x 0.05 = 0.1 g.0.1
- After. The mass does not change on dilution, so it is still 0.1 g.0.1
- Write the answer. 0.1
Check. 0.1 / 0.1 dm3 = 1 g/dm3. ✓
Method. Find the mass, then divide by the new volume.
- Mass. mass = 4 x 0.5 = 2 g.2 g
- New. concentration = 2 / 2 = 1 g/dm3.1
- Write the answer. 1
Check. The volume quadrupled, so the concentration fell to a quarter. ✓
Method. Divide by 5.
- Divide. 0.5 / 5 = 0.1.0.1
- Write the answer. 0.1
Check. 0.1 x 5 = 0.5. ✓
Method. Dilution does not change the moles.
- Moles. 0.1 x 0.25 = 0.025 mol, and this does not change on dilution.0.025
- Write the answer. 0.025
Check. The volume changes, not the moles. ✓
Method. Think about the solute.
- Solute. The mass of salt is unchanged.unchanged
- Conclude. It is spread through a bigger volume, so the concentration falls.bigger volume
- Write the answer. Accept any sentence that says the same mass of salt is spread through more solution.
Check. Concentration is mass per unit volume. ✓
Method. Think about what is added.
- Added. Only solvent (water) is added.only water
- Conclude. So the mass of solute is the same after dilution.same mass
- Write the answer. Accept any sentence that says only the volume increases; the solute is still there.
Check. Dilution changes the volume and the concentration. ✓
Chapter summary
| Skill | Method |
|---|---|
| Volume | 1 dm3 = 1000 cm3; divide cm3 by 1000 to get dm3. |
| Concentration (g/dm3) | mass of solute / volume in dm3. |
| Mass of solute | concentration x volume (dm3). |
| [H] Concentration (mol/dm3) | moles / volume (dm3); g/dm3 divided by Mr gives mol/dm3. |
| Reactions in solution | Concentration x volume gives moles of solute; the ratio gives moles of product; x Mr gives mass. |
| Dilution | Adding water: the mass or moles of solute stay the same; the concentration falls. |
Can you…?
- I can convert cm3 to dm3
- I can calculate the concentration in g/dm3 and the mass of solute
- I can use concentration to predict the mass of product in a reaction in solution
- I can explain dilution and its effect on concentration
- [H] I can calculate concentration in mol/dm3 and the moles of solute
Key terms
- solute
- the substance that dissolves in a solvent
- solvent
- the liquid that dissolves a solute, usually water
- concentration
- the amount of solute in a given volume of solution
- dilution
- adding solvent to a solution, which lowers its concentration
- mol/dm3
- [H] the number of moles of solute in one dm3 of solution
Exercise 9Z Check-up
Method. Divide by 1000.
- Divide. 500 / 1000 = 0.5.0.5
- Write the answer. 0.5
Check. 0.5 x 1000 = 500. ✓
Method. Divide by 1.
- Divide. 8 / 1 = 8.8
- Write the answer. 8
Check. Check: 8 g in 1 dm3. ✓
Method. Multiply 40 by 0.5.
- Multiply. 40 x 0.5 = 20.20
- Write the answer. 20
Check. Half of 40. ✓
Method. Convert, then divide.
- Convert. 250 cm3 = 0.25 dm3.0.25
- Divide. 10 / 0.25 = 40.40
- Write the answer. 40
Check. 0.25 x 40 = 10. ✓
Method. Divide the mass by the concentration.
- Divide. 5 / 20 = 0.25.0.25
- Write the answer. 0.25
Check. 0.25 x 20 = 5. ✓
Method. Divide the moles by the volume.
- Divide. 0.5 / 2 = 0.25.0.25
- Write the answer. 0.25
Check. 0.25 x 2 = 0.5. ✓
Method. Multiply the concentration by the volume.
- Multiply. 0.1 x 0.25 = 0.025.0.025
- Write the answer. 0.025
Check. 0.025 / 0.25 = 0.1. ✓
Method. Halve the concentration.
- Halve. 40 / 2 = 20.20
- Write the answer. 20
Check. 20 x 2 = 40. ✓
Method. Moles of copper sulfate, then mass of copper.
- Moles. 0.5 x 0.05 = 0.025.0.025
- Mass. 0.025 x 64 = 1.6.1.6
- Write the answer. 1.6
Check. Copper is 64 per mole. ✓
Method. Think about the volume.
- Solute. The mass of solute stays the same.same
- Volume. The volume increases, so the concentration falls.increases
- Write the answer. Accept any sentence that says the same mass of solute is spread through a bigger volume.
Check. Dilution lowers the concentration. ✓
Total 24 marks. Allow about 24 minutes.
Exercise 9X Exam-style questions
Method. Convert, then divide.
- Convert. 250 / 1000 = 0.25 dm3.0.25
- Divide. 10 / 0.25 = 40 g/dm3.40
- Write the answer. 40
Check. 0.25 x 40 = 10. ✓
Method. Multiply the concentration by the volume.
- Multiply. 25 x 0.4 = 10.10
- Write the answer. 10
Check. 10 / 0.4 = 25. ✓
Method. Divide the mass by the concentration, then convert.
- Divide. 3 / 15 = 0.2 dm3.0.2
- Convert. 0.2 x 1000 = 200 cm3.200
- Write the answer. 200
Check. 200 cm3 = 0.2 dm3, and 0.2 x 15 = 3 g. ✓
Method. Divide the moles by the volume.
- Divide. 0.1 / 0.25 = 0.4.0.4
- Write the answer. 0.4
Check. 0.4 x 0.25 = 0.1 mol. ✓
Method. Find the moles, then the mass.
- Moles. 0.5 x 0.05 = 0.025 mol.0.025
- Mass. 0.025 x 64 = 1.6 g.1.6
- Write the answer. 1.6
Check. Matches the worked example. ✓
Method. Divide the concentration by 4.
- Divide. 20 / 4 = 5.5
- Write the answer. 5
Check. 5 x 4 = 20. ✓
Method. Think about what is added.
- Added. Only solvent (water) is added during dilution.water
- Conclude. So the solute is still present in the same mass.same mass
- Write the answer. Accept any sentence that says only water is added, so the solute is still present.
Check. The volume and concentration change; the mass does not. ✓
Method. Find the moles, then divide by the volume.
- Moles. 4.0 / 40 = 0.1 mol.0.1
- Concentration. 0.1 / 0.25 = 0.4 mol/dm3.0.4
- Write the answer. 0.4
Check. Same as the 0.1 mol and 0.25 dm3 example. ✓
Method. Convert, then divide.
- Convert. 300 cm3 = 0.3 dm3.0.3
- Divide. 6 / 0.3 = 20.20
- Write the answer. 20
Check. 0.3 x 20 = 6. ✓
Method. Give the steps.
- Moles. Find the moles of solute: concentration x volume in dm3.moles
- Ratio. Use the balanced equation to find the moles of product, then multiply by Mr.ratio
- Mass. This gives the mass of product.mass
- Write the answer. Accept any answer that says find the moles of solute from concentration and volume, then use the balanced equation ratio to find the mass of product.
Check. The three steps are needed in order. ✓