Struggling with teaching mole ratios?
Updated: Nov 8, 2019
"But you had two of those ones, how can you only make one of those?"
"How do I get from 2 to 3?"
Students can go through the process of balancing equations, in my experience. And they can calculate moles of a reagent from a mass without much trouble either. But the mole ratio step in a calculation is a completely different ball game.
A large part of what this means is that students either (a) don't really understand what it means when we balance an equation, or (b) they just struggle with ratios per se.
I'm pretty sure that students find ratios mind-boggling. I think it underlies many of their difficulties in maths in general (fractions are the bane of many students' lives), but it is manifestly obvious in chemistry, as we use them so much.
On the other hand, this means that we have a unique opportunity to support students' understanding of the overarching ratio concept within our chemistry teaching.
Reacting mass calculations: What Not To Do
When I first started teaching, students were taught reacting mass calculations in Year 10 and mole ratios in Year 11. The reacting mass method discounts the ratio step by using the 'big numbers' as part of the Mr for the substance. Then students can just equate the 'moles' and convert back.
See the example above. Where do I even start...
Firstly, they are learning to calculate Mr incorrectly.
Secondly, they aren't actually calculating the number of moles of each substance when doing this, because they aren't using the actual Mr, but some horrible amalgam of moles and the ratio, embedding bad practice, which will need to be undone later.
Thirdly, by removing the mole ratio step, students are not being given access to the conceptual underpinnings of the entire mole concept.
Essentially the whole method is busy ensuring that students can show their teacher that they can 'do it' now (a serious case of right-answerism) but without embedding any actual understanding of moles for the students.
Fourthly, this method is incompatible with calculations involving solutions or volumes of gases, so students are going to have to learn different methods as soon as they encounter these. (There are also cheat methods for solution to solution and gas to gas, but interconversions between mass, concentration and volume have to go through moles).
When my students needed to learn the actual method the following year, results were disastrous. Unsurprisingly, I scrapped this after my first go at following the SOW.
Because when you add in the ratio step you suddenly get units! And Mr values! And moles! Which is pretty vital when you're teaching mole calculations.
Five Tips for Mastering Mole Ratios
But even if students have never been exposed to the ridiculous cheat method above, they still struggle with mole ratios. So here are my top tips for helping students to understand them:
1. Ensure students are totally comfortable mol=mass/Mr
Use simple mole calculations all the time: to determine masses for an experiment, to determine the moles of different reagents and see which one has more, to work out the Mr of an unknown, to calculate the empirical formula or water of crystallisation of a substance, etc. This ensures that when students eventually meet the mole ratio step, they can fluently use mole calculations without even thinking.
2. Embed other kinds of ratio first
By this I mean teach empirical formula way before mole ratios (and I mean way before, like a whole year before). This gives students an opportunity to meet ratios in a chemical context which is much less challenging. Student struggle conceptually much less with seeing the ratio between Cu and O in copper(I) oxide, and giving the formula, than with balancing equations. You can then move on to water of crystallisation ratios, which are another jump up as you use Mr instead of Ar. And again, use these in every topic until you are ready to teach the ratio in an equation.
This really helps students with (b): actually getting a better understanding of ratios, without any tainting of (a).
3. Lay out calculations properly
If there's one thing that gets my goat in chemistry, its poor maths. How many times do we see students put this equals that, and then get straight on with the next step by dividing, so that this no longer equals that? Argh... By laying it out properly, including units, and using notation for each line (my notation is shown in the example below), students are reinforcing their understanding (and picking up all of the marks).
These days I just refuse to mark calculations which don't have clear notation.
4. Investigate mole ratios in the lab
A couple of years ago, a question came up on an IGCSE paper which asked students to work out from the mass of product, which equation correctly represented the decomposition of potassium hydrogencarbonate. It really made students think about what the ratio signified in the equation. So this year, we investigated the same reaction in the lab. I designed a practical sheet, aiming to keep it as open ended as possible and make the students do all the thinking.
For the students who were struggling to access the task (we have mixed attainment groups) they could come and get a support sheet to scaffold the problem, but not do it for them.
This was a massive success. It took about an hour. I spent most of just observing (until the final analysis when I had to help a couple of students). And I got some really high quality work in. Obviously, feel free to use this practical with you students (the sheet is available HERE), or make up your own example (it's really easy as the students are doing all of the work).
This is all about putting the focus on (a): what does the balancing in the equation actually mean?
5. Get students to balance equations from mass data for reagents and products
For this you give students the mass of every reagent and product, and they use the data to calculate the moles, and then the ratio, and then write the balanced equation. They can then match this with how they might have balanced the equation if they just used the formulas. They can then have a go at some cracking reactions, or incomplete combustion reactions where there are multiple possible ways of balancing it from the formulas, to bet the equation which actually occurred.
And finally, supporting students with (a) and (b) at the same time: using ratios to determine the balancing.
Mole ratios are a difficult concept to master. No doubt about that. But with some forward planning and time to embed foundational skills, the path to mastery can be made a little less hazardous.