In the lab it comes out in a variety of ways. It comes out most commonly when the student gets to actually start doing their calculations and you ask them to relate that back to what they’ve actually physically measured. And when they start doing those sorts of things you realise there’s a bit of a misplaced idea here or a misconception that you can deal with there.
It’s something that needs to be reinforced, it’s not that you taught it in this unit for three weeks, we are over it. It’s something that keeps coming back, and that you can possibly reintroduce it, with not much change to your teaching. Not every single time, but every now and then remind the students, ‘remember, you still have to think about stoichiometry and limiting reagents’.
In some practical demonstrations it is very simple to see which reagent is limiting, for example if one of the reactants is liquid and you add just a tiny little bit of salt to it. Just from looking at it, which one do you think will be the limiting reagent here? We have 20ml of A, and we’re going to add half a gram of B. We know though that the number of moles is the important thing, but sometimes it works just to illustrate the concept.
If you teach a course that then goes on to other things like equilibrium, electrochemistry, intermolecular forces etc, make sure that all your equations are balanced. Because it can get very lazy, especially when you’re doing organic chemistry, you don’t bother balancing anything. Coming back to that concept each time is really key for them if they’re going to understand the stoichiometry.
This concept is something that needs to be reinforced. It’s not that you taught it in this unit for three weeks, we are over it. It’s something that keeps coming back, and you can possibly reintroduce it, with not much change to your teaching. Not every single time, but every now and then remind the students, 'Remember, you still have to think about stoichiometry and limiting reagents.'
Use the units to check that they’ve actually done the right calculation. The other thing is when they’ve got the mole concept and they’re applying it, try to get students to do an estimation of the answer. So are they estimating something that’s very large?
They’re stepping into an area where they must do calculations and they must support them with strictly mathematical algebra, so that when they’re doing calculations it’s clear what the data is and what the units are and how these all fit together to make up the calculation they’re doing. So it’s really about making things mathematically strict.
Use clicker questions in a lecture. Use multiple choice questions and you can see the class distribution, that is, what percentage of students got it right. If a lot of them don’t get it right, go through it again. That really helps to get the feedback from students about their understanding. Use it for other topics as well but this will be one topic to use it for in a big class.
