What is a mole?
No, not that type of mole!
This post is most suited to those of you studying GCSE Chemistry and confused about the topic of moles, which is within the area of Chemistry known as quantitative chemistry.
As a private tutor, I have become more familiar with the trickier topics within science and maths and occasionally have to brush up on the easier topics which is a little topsy-turvy, but this is because I help students with the topics they need help with and as such I get a lot of practice teaching the harder topics and the easier topics get neglected a little. One of these topics which I have become extremely familiar with is moles. It has caused so many students so much difficulty that I have now taught it countless times, and so not only do I understand it very well myself, but I also have started to notice some of the reasons why it is so confusing.
Balanced equations
Let us start with a simple balanced formula equation for iron reacting with chlorine to form iron chloride. If you want a real challenge cover the equation below and see if you can write it out and balance it, otherwise take a peak.
2 Fe + 3Cl2 → 2FeCl3
Firstly, it is worth mentioning that the following sentence is not true:
“2 atoms of iron reacts with 3 molecules of chlorine to form 2 molecules of iron chloride.”
Why not? Seems right, doesn’t it? Iron chloride is an ionic compound and ionic compounds form ionic lattices, not molecules.
However, we can rewrite the sentence in terms of moles:
“2 moles of iron reacts with 3 moles of chlorine to form 2 moles of iron chloride.”
This is fine. However, we are getting ahead of ourselves now. I haven’t answered the question at the top of this blog yet! What even is a mole??
Enter Avogadro
Lorenzo Romano Amedeo Carlo Avagadro (yes, that is one person’s name!) came up with a crazy idea that you could weigh a bunch of stuff and calculate how many atoms or molecules were in that bunch of stuff. After doing a bit of science… he, and some accomplices, came to up with a very special number which became known as Avagadro’s Number. This number is 6.02x1023 or 602,252,000,000,000,000,000,000. Essentially, a mole of something contains this many atoms/molecules.
This is worth repeating.
A mole of carbon is 6.02x1023 carbon atoms.
A mole of iron is 6.02x1023 iron atoms.
A mole of water is 6.02x1023 water molecules.
A mole of hydrogen is 6.02x1023 hydrogen molecules.
The last one catches people out a lot. People often reason that because hydrogen is an element you treat like the first two examples and count atoms rather than molecules. Water is a covalent compound and so of course you count molecules, but hydrogen also exists as molecules, not atoms. This is the same for most non-metal elements excluding noble gases. So, when you’re considering what a mole of X is, you need to consider how X exists before assuming it is a mole of X is 6.02x1023 atoms of X.
Why this number though??
Basically, the number Avagadro came up with his very important because a mole of carbon weighs 12g and strangely enough 12 is also carbon’s atomic mass on the periodic table. This is no coincidence. This is the same for any element. The mass of one mole, known as the molar mass can be easily found by simply looking up the mass number of that element. To check you understand this concept, have a go at these 3 questions and cover up the answers after the questions:
What is the molar mass of potassium?
What is the molar mass of calcium?
What is the molar mass of nitrogen?
You should have got 39g, 40g and… 28g! Remember what I said about molecules? Nitrogen exists as molecules and so when I say a mole of nitrogen I mean a mole of nitrogen molecules. The mass number of nitrogen is 14, so a mole of nitrogen atoms would be 14g, but because the formula for nitrogen is N2 like most non-metal gases (other than noble gases) then a mole nitrogen molecules would be the equivalent to two moles of nitrogen atoms.
This is part of a 3 part series on moles. The next post will be looking at an example calculation in which you will have to calculate the expected mass of a product produced in a chemical reaction by analysing the balanced equation and calculating the molar masses of the substances in question. The third post will be looking at a titration calculation.
I hope you find this how-to format of blog posts helpful. If so, please share widely with anyone you feel would benefit.
I look forward to seeing you in the next post!
