Calculating and comparing solution concentrations | 16-18 years

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

1. Issue ‘traffic light’ cards to all students.
2. 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’.
3. Pour 100 cm³ 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.
4. 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.
5. After adding water to beaker C to make the total volume 100 cm³ again, ask students for a second time to give their view on:
   1. The number of moles of copper(II) sulfate in beakers B and C.
   2. The concentration of copper(II) sulfate in beakers B and C.
6. Use their indications as an aid to sharing the learning objective.

Explaining concentrations

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

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

Card matching: stage 1

1. Move students back into pairs.
2. Give a set of ‘Concentration cards’ to each pair and an ‘Answers’ sheet to each student.
3. 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:

1. 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.
2. Join up with another pair.
3. Exchange the cards they have devised.
4. Match up and record the cards devised by the other pair on their ‘Answers’ sheet.
5. 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 SO₄^2-, when the following three solutions are mixed together to give a total volume of 2 dm³:

• 1000 cm³ of 0.1 mol dm^-3 HCl
• 500 cm³ of 0.2 mol dm^-3 NaCl
• 500 cm³ of 0.2 mol dm^-3 Na₂SO₄

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 cm³, x3

Chemicals

• Copper(II) sulfate solution 0.1 mol dm^-3, 200 cm³
• Water, 50 cm³

Health, safety and technical notes

• Read our standard health and safety guidance.

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 dm3
0.4 mol dm-3 NaOH	4 g	250 cm3
0.5 mol dm-3 NaOH	40 g	2.0 dm3
2.5 mol dm-3 Na+ ions	10 g	100 cm3

Extension activity

Total volume = 2 dm³ (ie 2000 cm³)

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 Na₂SO₄ – ie 0.10 mol dm^-3 Na⁺ and 0.05 mol dm^-3 SO₄^2-

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 SO₄^2- = 0.05 mol dm^-3