Wednesday, June 28, 2017 0

How many moles of air are in your lab?

What mass is this air?

These are a couple of great questions you can set your students and ask them to come up with some answers using the ideal gas equation.

You will need a barometer (to determine the air pressure), a thermometer (to determine the temperature of the lab) and a rule to calculate the volume of the lab and hopefully some imaginative students.

I carried out this work with my class a while back and have been meaning to write about it. What I found interesting when I carried out this task was how close the students results were to each other.

Image sourced: By Zuzanna K. Filutowska – Own work, CC BY-SA 3.0 https://commons.wikimedia.org/w/index.php?curid=29764366

So how did they do it?

Well, they calculated the number of moles of air in the lab using the ideal gas equation – that was relatively straight forwards.

To get the mass of air the students made the assumption that the air in the lab was 21% oxygen, 78% nitrogen and 1% argon (see  http://climate.ncsu.edu/edu/k12/.AtmComposition)

This meant that if, for example, they had found out there was 258.9 moles of air, 54.37 moles of it would be oxygen, 201.9 moles would be nitrogen and so on. After that it was a relatively easy enough to work out the mass of this number of moles of gas.

As written above, student results were remarkably close. The variable that gave the most uncertainty for me was measuring the volume of the lab, do you take the lab as being a ‘hyperrectangle’? (that’s the name for a rectangular cuboid! (https://en.wikipedia.org/wiki/Hyperrectangle)  or do you try to take into account those little deviations, for example, my windows go outwards, adding volume to the hyperrectangle. Some students attempted to take this into account where as others didn’t – it turns out that the percentage extra they added was only minimal and so it wasn’t really worth the effort trying to make every extra volume (or lack of volume) count.

It did make me think though. As IB teachers, we should be trying to link out course with other subjects that students study and here is a great example of linking chemistry with math. Is it should be possible, for example, to calculate the mass of air in a hot air balloon. I won’t pretend I know the name of the shape of the hot air balloon or how to calculate the volume of it – this is where I would need to lean on my math colleague but it should be possible to do.

Do you have any examples of cross curricular activities / projects? If so, I’d love to read about them below.