Determining unknown concentration | Johnstone’s triangle

The volume of the acid of unknown concentration can be determined experimentally using a practical technique called titration.

Learning objectives

1. Recognise and read volume from titration apparatus.
2. Calculate the number of moles of sodium hydroxide.
3. Calculate the concetration of hydrochloric acid with unknown concentration.

How to use Johnstone’s triangle

Use Johnstone’s triangle to develop learners’ thinking about scientific concepts at three different conceptual levels:

• Macroscopic:  what we can see. Think about the properties that we can observe, measure and record.
• Sub-microscopic:  smaller than we can see. Think about the particle or atomic level.
• Symbolic representations:  Think about how we represent chemical ideas including symbols and diagrams. 

For learners to gain a deeper awareness of a topic, they need to understand it at all three levels.

When introducing a topic, do not try to introduce all of the levels of thinking at once. This will overload working memory. Instead complete the triangle over a series of lessons, beginning with the macroscopic level and introducing other levels, in turn, once secure.

All of the levels are interrelated, for example, learners need visual representation of the sub-microscopic in order to develop mental models of the particle or atomic level.

Scaffolding 

Share the structure of the triangle with learners prior to use. Tell them why you are using it and how it will help them to develop their understanding. Use an ‘I try, we try, you try’ approach when introducing Johnstone’s triangle for the first time. 

More resources 

To further develop learner’s thinking in all areas of Johnstone’s triangle, try our Developing understanding of concentration and moles worksheet. This includes icons in the margin referring to the conceptual level of thinking needed to answer the question.