Monday, March 21, 2016 0

Moles, as I hope you realise by now are very useful to chemists. The mole is just a concept, an idea or a number (much like a dozen means 12) that allows us the measure out the right ratios of atoms, molecules, ions, etc.

The mole is so useful that we often forget that when we ‘take 0.25 moles of magnesium’ we are actually measuring out a certain number of atoms of magnesium – we use the expression ‘mole’ so freely, we often forget this.

To measure out a specific number of moles of a substance you need to take its mass and divide it by its relative molecular or atomic mass.

For example, following on from the above idea, with 0.25 moles of magnesium, we would measure out 6g (as the relative atomic mass of magnesium is 24*) and 6 / 24 = 0.25 moles.

* Some rounding has occurred here as the relative atomic mass of magnesium is, according to the IB data book 3rd edition 24.31 g mol -1

What I want you to think about though is the 6g, ie, the substance you (as I deliberate phrased) ‘measure out’.

When we ‘measure out’ the substance we weigh it – but are we needing its mass (ie, the amount of ‘stuff’) or the weight (the force caused by gravity)?

The answer should be obvious (I hope) we need the mass.

Mars

Image sourced from: https://commons.wikimedia.org/wiki/File:Mars_23_aug_2003_hubble.jpg

Now, if I ask my class to take 0.25 moles of magnesium, they put it on an electronic balance (as I am sure you do) and it should tell them 6g (or there about) – but what would they do if they were lucky enough to be on Mars?

The gravity on Mars is only approx 40% of that on Earth. So if they needed 0.25 mol of magnesium on Mars, the electronic balance would actually tell them to measure out 16g of magnesium **, even though it would only read 6g.

** Have I scaled this correctly? 40% of 16g = 6g, the balance would read 6g but in reality there would be 16g measured out

The implications for this are huge – what about living on Mars and cooking – how would you do this? You would find yourself cooking too much! How would you overcome it?

Well, I needed my physics colleagues to help me out – and I was amazed at the simplicity of their answer – all you need is a mass balance – what I would call a weighing scales – not an electronic balance but something where you have known masses on one side and you balance them with the substance you are measuring out. You would have a comparison.

It is a case of the new technology (ie, the electronic balance) being less useful than the old – which in itself is a great example of Occam’s razor (NOS) – it is also a great ‘thought’ experiment to try on your friends and teachers.

That said, how would they measure the 0.25 mol on the ISS? This orbits the Earth under zero gravity ….. it would be great to read your ideas to this one.

Disclaimer – I am a chemistry teacher by trade – I enjoy physics and find it an interesting subject but could now way teach it at IB level. Therefore, if you study physics, as well as chemistry and find this article superficial or that it contains the wrong terminology (or ideas) then I apologise for my ignorance!