Practise calculating the concentration of a solution from the mass of solute and the volume of water using this lesson plan with activities for 16–18 year olds

In this activity, students begin by comparing the concentrations of several solutions and reflecting on their current level of understanding. They then work in pairs using cards to link a mass of solute, volume of water and concentration of a solution, exploring the process of calculating concentration.

The lesson plan includes an extension activity giving students additional practise calculating the concentration of solutions.

## Learning objectives

Students will understand:

- How to calculate the concentration of a solution.

## Sequence of activities

### Introduction and demonstration

- Issue ‘traffic light’ cards to all students.
- Explain that they are going to:
- Compare the concentration and number of moles of solute in solutions.
- Use the ‘traffic light’ cards to indicate their view: green for ‘the same’, red for ‘different’ and yellow for ‘unsure’.

- Pour 100 cm
^{3}of copper(II) sulfate solution into each of two beakers A and B. Pour half of the solution from beaker A into a third beaker C. - Ask the students to give their view on the following comparisons:
- The number of moles of copper(II) sulfate in beakers B and C.
- The concentration of copper(II) sulfate in beakers B and C.

- After adding water to beaker C to make the total volume 100 cm
^{3}again, ask students for a second time to give their view on:- The number of moles of copper(II) sulfate in beakers B and C.
- The concentration of copper(II) sulfate in beakers B and C.

- Use their indications as an aid to sharing the learning objective.

### Explaining concentrations

Give each student an ‘Explaining concentrations’ sheet. Organise the students to:

- Work individually to complete the explanations and the ‘can do’ / ‘can’t do’ / ‘not sure’ boxes.
- Join with another student.
- Compare responses and convert any ‘can’t do’ or ‘not sure’ responses to ‘can do’.
- Join with another pair of students if there are still any ‘can’t do’ or ‘not sure’ responses.

### Card matching: stage 1

- Move students back into pairs.
- Give a set of ‘Concentration cards’ to each pair and an ‘Answers’ sheet to each student.
- Circulate and support with prompts while pairs of students:
- Group cards together showing the mass of sodium hydroxide and volume of water needed to produce the concentration shown on one of the cards.
- Record their answers on the ‘Answers’ sheet.
- Explain the general approach to calculating concentrations (on the ‘Answers’ sheet).

### Card matching: stage 2

When pairs have recorded and shown the correct answers, give them a set of ‘Blank Concentration cards’ and a solute chosen from:

- Sodium carbonate.
- Sulfuric acid.
- Potassium hydroxide.
- Calcium bromide.
- Copper(II) sulfate.

Circulate and support with prompts while pairs:

- Devise their own set of concentration cards using the solute given to them so that all cards are used up when the mass of solute, volume of water and concentration of solute or ions in solution are matched up.
- Join up with another pair.
- Exchange the cards they have devised.
- Match up and record the cards devised by the other pair on their ‘Answers’ sheet.
- Help each other pair to select appropriate cards where this is necessary.

### Extension activity

As an extension, set the following problem and work through the solution in a plenary.

#### Problem

Calculate the final concentrations in mol dm^{-3} of H^{+}, Na^{+}, Cl^{-} and SO_{4}^{2-}, when the following three solutions are mixed together to give a total volume of 2 dm^{3}:

- 1000 cm
^{3}of 0.1 mol dm^{-3}HCl - 500 cm
^{3}of 0.2 mol dm^{-3}NaCl - 500 cm
^{3}of 0.2 mol dm^{-3}Na_{2}SO_{4}

### Before finishing

Give each student a ‘Review’ sheet to complete and hand in.

### Feedback

Give written feedback that acknowledges achievement and leads students to recognise their next steps and how to take them.

## Commentary

The snapshot of student confidence, at the start of the session, gives the students a baseline as well as informing the teacher.

By writing explanations of how to do simple calculations and discussing their competence in a structured way the students are helped to recognise their own strength and weaknesses. Their learning is embedded when they set a further card matching exercise for their peers.

The final review guides students through an assessment that will reinforce confidence and help them to interpret feedback from the teacher.

## Practical notes

### Apparatus

- Beakers, 250 cm
^{3}, x3

### Chemicals

- Copper(II) sulfate solution 0.1 mol dm
^{-3}, 200 cm^{3} - Water, 50 cm
^{3}

### Health, safety and technical notes

### Other equipment

- A set of ‘traffic light’ cards for each student

## Answers

### Card matching

Concentration | Mass of NaOH | Volume of water |
---|---|---|

4 g dm^{-3} NaOH |
2 g | 0.5 dm^{3} |

0.4 mol dm^{-3} NaOH |
4 g | 250 cm^{3} |

0.5 mol dm^{-3} NaOH |
40 g | 2.0 dm^{3} |

2.5 mol dm^{-3} Na^{+} ions |
10 g | 100 cm^{3} |

### Extension activity

Total volume = 2 dm^{3} (ie 2000 cm^{3})

Assume all species are strong electrolytes and are fully dissociated in aqueous solution.

Final solution contains:

- 0.05 mol dm
^{-3}HCl – ie 0.05 mol dm^{-3}H^{+}and 0.05 mol dm^{-3}Cl^{-} - 0.05 mol dm
^{-3}NaCl – ie 0.05 mol dm^{-3}Na^{+}and 0.05 mol dm^{-3}Cl^{-} - 0.05 mol dm
^{-3}Na_{2}SO_{4}– ie 0.10 mol dm^{-3}Na^{+}and 0.05 mol dm^{-3}SO_{4}^{2-}

Therefore:

- Concentration of H
^{+}= 0.05 mol dm^{-3} - Concentration of Cl
^{-}= 0.05 + 0.05 = 0.10 mol dm^{-3} - Concentration of Na
^{+}= 0.05 + 0.10 = 0.15 mol dm^{-3} - Concentration of SO
_{4}^{2-}= 0.05 mol dm^{-3}

### Downloads

#### Explaining concentrations sheet

**Editable handout**| Word, Size 58.5 kb#### Explaining concentrations sheet

**Handout**| PDF, Size 41.61 kb#### Answers sheet

**Editable handout**| Word, Size 59 kb#### Answers sheet

**Handout**| PDF, Size 41.73 kb#### Review sheet

**Editable handout**| Word, Size 50.5 kb#### Review sheet

**Handout**| PDF, Size 37.93 kb#### Concentration cards

**Editable handout**| Word, Size 56 kb#### Concentration cards

**Handout**| PDF, Size 40.31 kb#### Blank concentration cards

**Editable handout**| Word, Size 55.5 kb#### Blank concentration cards

**Handout**| PDF, Size 38.14 kb

### Additional information

This lesson plan was originally part of the Assessment for Learning website, published in 2008.

Assessment for Learning is an effective way of actively involving students in their learning. Each session plan comes with suggestions about how to organise activities and worksheets that may be used with students.

**Acknowledgement**

K. Crawford and A. Heaton, *Problem solving in analytical chemistry, *Section 1*, Calculating concentrations*. London: Royal Society of Chemistry, 1999.

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