PRE IB: The Basics – Moles

This blog was written by Dave Allen, an experienced IB Chemistry teacher. To read more Chemistry blogs for students and teachers, click here.

Yes, you have read that correctly – we get moles in Chemistry as well as Biology. However, our mole is not the small furry type – it’s a number!

You see, in Chemistry we count things not the obvious way (i.e., 1, 2, 3, 4, 5 and so on) but by using a word – the ‘mole’ – to represent a number. Clear?

Imagine not be allowed to use a number to describe a quantity. Let’s use an example you are familiar with, imagine you had to use the word ‘dozen’. So every time you wanted a number of something, you would need to express it in terms of a dozen. For example, you want to buy 2 kg of apples, ‘2’ is a number. You would need to ask for 2/12 (two-twelfths) of a dozen of apples or 01.666 reoccurring dozen apples. It seems complicated but that is just what we do in chemistry.

Except for one thing, our number is not 12 but 6.02 x 10 23! This is a huge number. Imagine being able to go back to the start of time, the big bang and being able to count to one million per second. You would hit 6.02 x 10 23 around now. 602000000000000000000000 is a very large number! But why so large? Well, it really reflects the facts that atoms, molecules or ions are so small.

The next logical question is how do we actually count such a large number? The answer is surprisingly simple. We weigh them. Let’s use another real world example. If you went to the bank with a bag of 1 penny coins, the cashier would count them very very quickly. The cashier would know the mass (weight) of one coin, so could very quickly work out the total mass and hence the number of coins. Well, we do exactly the same in Chemistry.

Not these kind of moles…

And why do we weigh atoms? The answer has actually been covered above – it is because they are so small.

So how does all of this come together?

The number of moles = mass / molecular mass

The number of moles can be thought of as the word describing the number of atoms.

The mass, well, that’s straight forward, it is the weight!

The molecular mass – this is the larger of the two numbers you will find for an element in the periodic table. For example, for Lithium it is 7 (or 6.94, depending on which periodic table you use). To get the molecular mass for say, water, you need to know the formula (H2O) and add up the respective individual numbers (two H’s, one O).

Do you have any questions about moles? If so, I’d love to hear from you so please post your question(s) below.

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