Deepen learners’ understanding of concentration in mol/dm3 with Johnstone’s triangle
Developing understanding is a series of resources that encourages learners to connect their thinking at the macroscopic, sub-microscopic and symbolic levels.
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A ramped worksheet to help learners develop their mental models of concentration and mass. With icons to indicate the conceptual level/s of each question.
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Learning objectives
- Determine the concentration of a solution in mol/dm3 from a given number of moles of solute and a volume of 1 or 2 dm3 of water.
- Determine the number of each ion in a solute that are present in a solution with concentration 1 mol/dm3.
- Interpret particle diagrams of solutions in terms of water molecules and solute ions.
- Calculate the number of moles of solute from the volume (in cm3) and concentration of solution.
- Calculate the concetration of sodium hydroxide solution from the volume of hydrochloric acid that reacts exactly with a given volume.
How to use this resource
This resource aims to develop learners’ understanding of concentration in mol/dm3. The questions encourage learners to think about what concentration means at the sub-microscopic level as well as how it is calculated. As a result, learners should develop more secure mental models to support their thinking about this topic.
- When to use? Use after initial teaching or discussion of this topic to develop ideas further. You can also use as a revision activity.
- Group size? Suitable for independent work either in class or at home. Or use the questions for group or class discussions.
- How long? 15–30 mins
Johnstone’s triangle
Johnstone’s triangle is a model of the three different conceptual levels in chemistry: macroscopic, symbolic and sub-microscopic. You can use Johnstone’s triangle to build a secure understanding of chemical ideas for your learners.
Introduce learners to Johnstone’s triangle with our Concentration of acids Johnstone’s triangle worksheet or our Determining unknown concentration Johnstone’s triangle worksheet.
Further reading
Read more about how to use Johnstone’s triangle in your teaching with these articles:
- Develop deeper understanding with models
- Improve students’ understanding with Johnstone’s triangle
- Practical ideas for using Johnstone’s triangle
Norman Reid’s book The Johnstone triangle: the key to understanding chemistry provides an more in-depth overview, the first chapter is available to read online.
Support
This worksheet is ramped so that the earlier questions are more accessible. The activity becomes more challenging in the later questions. You can give extra explanations for the more challenging questions. If completing as an in-class activity, it is best to pause and check understanding at intervals, as often one question builds on the previous one.
It is useful for learners to observe macroscopic properties first-hand. You could circulate examples of substances in the classroom, run a class practical of a chemical reaction or show a teacher demonstration of properties.
Give learners physical models to use and manipulate, such as a Molymod kits or counters.
Additional support may be needed for any learners still lacking in confidence in the required symbolic representation, for example by sharing and explaining a diagram or a simulation that can show movement of the particles.
Answers and guidance
There are five multi-part questions in the student worksheet. Answers can be found in the teacher notes.
The first question develops learners’ understanding of the expression of concentration with the units moles per dm3 (mol/dm3). Although a mole represents a number of sub-microscopic particles, in this question the focus is on a mole as a quantity of the solute (macroscopic understanding).
In question two learners develop understanding of how the concentration of a solution in mol/dm3 connects to the number of moles of each ion (sub-microscopic understanding) in the formula (symbolic understanding) of the solute. This question also supports learners to connect mass (macroscopic understanding) to number of moles. This question uses the term ‘formula unit’ to describe the basic number of ions shown in a chemical formula. For example, the formula unit of MgO would be one magnesium ion and one oxide ion and the formula unit of MgCl2 contains one magnesium ion and two chloride ions. The focus is on interpretation of the formula rather than on the term formula unit.
Question three develops learners’ understanding of how to calculate concentration in mol/dm3 (macroscopic understanding). It then supports learners to understand particle diagrams of a solution (symbolic understanding) in terms of water molecules and solute ions. This question supports learners to convert from cm3 to dm3 but assumes prior familiarity with dm3 as a unit of volume.
In question four learners develop understanding of how to calculate the number of moles (sub-microscopic understanding) of solute from the volume (in cm3) and concentration of a solution.
In the final question develops learners’ understanding of how the balanced chemical equations (symbolic understanding) provide the relationship between the number of moles in a given volume of an unknown concentration of alkali (sub-microscopic understanding) and the number of moles of acid that react exactly with it. Learners are then supported to use this information and the volume of acid that has reacted with the alkali (macroscopic understanding) to calculate the concentration of the alkali. This question assumes familiarity with the quantitative meaning of a balanced chemical equation in terms of the ratio of moles of reactant that react. Familiarity of the experimental process of titration is not expected.
Downloads
Concentration and moles developing understanding student sheet
Handout | PDF, Size 0.33 mbConcentration and moles developing understanding teacher notes
Handout | PDF, Size 0.29 mbConcentration and moles developing understanding student sheet
Editable handout | Word, Size 0.76 mbConcentration and moles developing understanding teacher notes
Editable handout | Word, Size 0.51 mb
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