A Modelling Chemistry approach to molarity

With my Year 11’s, we’ve recently been working through the idea of molarity; describing the concentration of a solution in terms of the moles of solute it contains, rather than just mass. I wanted the students to realise the need to describe how much of a substance there is in a solution, especially when we want to compare solutions of different substances.

To this end, I wanted to use a Modelling Chemistry approach, getting the students to work out what is happening at the particle level in solutions of different concentration. The Modelling Chemistry program is a teaching approach that I’ve really wanted to try out for a long time now. I would best describe it as a guided inquiry approach, somewhat of a middle ground between pure direct instruction and pure open-ended inquiry learning. Corey put it really well:

Now this is where I think modeling makes a great alternative. In modeling the teacher serves as guide, suggesting a simple model of the world to start. Then as other options for explaining phenomena arise, they take a time to test them and see if the current model makes sense. After options are discussed or explored, an adjustment is decided up on with the class and the class moves forward. By doing this, student follow the great discoveries in chemistry understanding why we describe the world as we do. And instead of having to take our word for it, they get to experience why our models work and see that we never have a complete picture, just an ever improving view of the world.

I’m still just getting my head around the way that I might implement Modelling Chemistry in my classroom, but I figured that this approach would serve me well with this area. So we did the following.

As a primer in the previous lesson, I asked them to rank six copper sulfate (CuSO4) solutions from the most particles to the fewest particles, based on visual alone. Below is a bit of a snapshot of what we ended up with:

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In the next lesson, I prepared two solutions of copper sulfate, one with one teaspoon of solute and the other with two teaspoons. I asked the students to describe the number of particles in each solution – they identified that the one with two tsp had more particles, because we added more of the solid to start with.

Then I took the CuSO4 solution with one tsp and made up a solution with one tsp of sodium carbonate (Na2CO3) and put them next to each other. I asked the students which one had more particles and the blank looks I got! I asked them to work out what they would need to know and do in order to work it out for sure. They decided that we needed to know exactly what mass of the solutes were added at the start, but to work they out we’d need to evaporate them first.

So we got out the Bunsens and evaporating basins and dried them out 50 mL samples, measuring the mass of solid left behind and averaging out our measurements. From there, they worked out the number of grams in 50, 100 mL and then 1 L, and then the number of moles of each in the same volumes. This really helped them to see that, even though the teaspoons of each substance were similar in volume, the sodium carbonate contained more particles and therefore more moles, because its molar mass was smaller:

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So to finish it all off, we moved towards calculating concentration using c = n/V and having a common unit of concentration in moles per litre.

I’m really proud of this lesson, because a) the students came up with the ideas themselves, b) they arrived at the need for comparing different substances by some common standard all on their own, and c) they saw the links between the number of particles, moles and concentration much more quickly. It really helped to show me the value of the Modelling approach, as it really helped the students to build their understanding from the particle level up. We followed this up in the subsequent lesson with the Mistake Game, but that’s for the next post.

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