60 
Concentrations 
Post 16 

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 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
and after adding water to beaker C to make the total volume 100 cm^{3} 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 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}


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


Explaining concentrations sheet 


Answers sheet 


Review sheet 
 Set of Traffic light cards
For each pair of students


Set of Concentration cards 


Set of Blank concentration cards 
Equipment
For the demonstration
 Three 250 cm^{3} beakers
 200 cm^{3} copper(II) sulfate solution 0.1 mol dm^{3}
 50 cm^{3} water.
Answers
Concentration card activity
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}
Acknowledgement
K. Crawford and A. Heaton, Problem solving in analytical chemistry, Section 1, Calculating concentrations. London: Royal Society of Chemistry, 1999.
