60 Concentrations Post 16 Working in groupsSelf assessmentPeer assessmentSharing objectives and criteriaQuestioningUsing feedbackUsing tests

In this activity, students select cards to link a mass of solute, volume of water and concentration of a solution. They explore and discuss the process of calculations in chemistry.

Learning objectives

Students will understand:

  • how to calculate the concentration of a solution.

Sequence of activities

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
    Yellow for unsure

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.

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

and after adding water to beaker C to make the total volume 100 cm3 again,

  • 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.

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

  • firstly work individually
  • 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.

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).

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.

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

Calculate the final concentrations in mol dm-3 of H+, Na+, Cl- and SO42-, 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

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

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

Assessment for learning 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.

Resources

Props

For each student

Download Word Download PDF Explaining concentrations sheet
Download Word Download PDF Answers sheet
Download Word Download PDF Review sheet
  • Set of Traffic light cards

For each pair of students

Download Word Download PDF Set of Concentration cards
Download Word Download PDF Set of Blank concentration cards

Equipment

For the demonstration

  • Three 250 cm3 beakers
  • 200 cm3 copper(II) sulfate solution 0.1 mol dm-3
  • 50 cm3 water.

Answers

Concentration card activity

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 SO42-

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 SO42- = 0.05 mol dm-3

Acknowledgement

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