© 2017 by Emily Rose Seeber. 

Moles

Blogs about Chemistry

The mole is generally thought to be a 'threshold concept' in Chemistry. Once students get it, there are all kinds of problems they can understand which were previously denied to them; but without the mole, their understanding of Chemistry will never progress past classifying substances and spotting patterns. ​​

Basically, the issue is that the mole isn't really 'content' to a chemist, but a 'tool'. Yet we have historically taught the mole as a topic in Chemistry; as if it is content. But it is not. And I think it is this mismatch which is at the root of students' perpetual struggle with the mole. They forget the equations, they struggle to use mole ratios, they don't know when to use each specific type of mole calculation (even though to a real chemist, all the different 'types' are actually the same). 

 

I have been struck by two key points: firstly, students need to learn when to use a particular tool as well as how to, so teachers need to interleave problems together so that students can learn this; and secondly, that when teaching abstract concepts they need to be met multiple times from different perspectives.

 

So I completely restructured the way we teach moles calculations so that there is something in each term over the course of the IGCSE from that topic, and each time the additional new content is used in context, as a tool, not as new content.

 

Each of the new aspects of the calculation has a distinct practical context which is taught together with the calculation technique to provide another perspective. In the table I have listed one example for each, but in reality we use many more, although some might be demonstrated, and others might be analysing the evidence from a practical write-up rather than the students doing an investigation themselves. (At least one student practical for each of the aspects below.)

How does it work in practice?

So, after students learn about how to calculate an empirical formula from practical data, they then use this technique in a wide variety of different situations before they learn something different about the concept of the mole. They are expected to be able to use this technique fluently as part of their standard set of skills in Chemistry.

 

I now teach ionic bonding from the perspective of being investigative scientists. The students calculated empirical formulae of sodium chloride, magnesium chloride, and aluminium chloride and spotted the patterns themselves. We then did the same for sodium chloride, sodium oxide, and sodium nitride. The question sheet is available HERE

I try to ensure that students contextualise their understanding of moles with practical work. Some resources are below:

  • A practical for teaching calculations involving gases is available HERE

 

In between these key identified points at which students are learning a new technique, previously learned moles content is woven into their other topics so that students are always revising the methods and when they should be used in a range of contexts. Here are some examples:

  • A worksheet which uses relative atomic mass calculations, moles calculations and empirical formulae is available HERE.

  • A worksheet for calculations involving gases from the Triple Award Chemistry course is available HERE

 

Students are then supposed to find moles as a standard part of their chemical toolkit, like being able to evaluate an experimental procedure, rather than a completely separate kind of thing they do for “the moles question in the exam”.