The Mole

Not this kind Or this kind

But First, An AnalogyIf I ask you to go get 3 dozen eggs, that’s 36 eggs.

1 dozen = 12

This is not exactly a unit; it’s a grouping that modifies a unit.

I still need to tell you 3 dozen eggs, or 2 dozen bagels; eggs and bagels are the actual unit.

All I’ve done is put them into groups of 12 to make counting easier.

Back to the MoleA mole is a grouping just like pair (2), or dozen (12)

1 mole = 6.022*1023 (this is called Avogadro’s number)

Ok, so that’s a lot of stuff.

We can have a mole of anything: bagels, eggs, walruses.(that’s a lot of walruses)

We can have two moles of bagels, in which case we have 1204400000000000000000000 bagels (12.044*1023)

Probably, this is not a useful grouping for bagels, eggs, or really much of anything else you can see.

Definitely not Walruses

Physics on a mole of moles: http://what-if.xkcd.com/4/Walruses would presumably be about the same.

But What About Atoms?

This is what one mole of carbon atoms (graphite allotrope) looks like.

Counting out atoms or molecules in groups of moles is a lot more reasonable.

(In dozens, this is still 50183000000000000000000 dozen atoms. Dozens will not be a very useful grouping here)

Doing Math With Moles

Yes, you can have decimals

Moles Are Not Conserved

Moles Are Not ConservedBecause it makes 1 mole of molecules. We have grouped the atoms together into molecules, then counted how many of those groups we have.

It’s easier to see with pairs:

1 pair of CO2 =

Which is 1 pair of C, and two pairs of O:

So unlike mass/energy, which can neither be created nor destroyed, moles are not conserved. If you start with 3 moles of something, you don’t have to end up with three moles, because you may have chosen to group the things differently.

So 1 mole of CO2 is 3 moles of atoms, or 2 moles of oxygen with 1 mole of carbon. It all depends on how we group it.

So Why This Awful Number?Why 6.022*1023?

Wouldn’t 1*1023 or 1*1024 make more sense, and have the advantage of being metric?

YES! But…

So Why This Awful Number?Mass of a proton/neutron = 1.6726*10-24 g

Mass of 1 mole of protons/neutrons:

6.022*1023 protons * 1.6726*10-24 g/proton = 1 g !!!

I bet this will turn out to be relevant….

Why Moles At All?1. Numbers of atoms are so huge that even scientific

notation gets annoying. Writing “2 moles Na” is much simpler than “1.2044*1024 atoms Na”

2. That thing about 1 mole of protons weighing 1 gram

This is a mole cricket. Real thing. I promise. Totally not made up.

Summary• Moles are a grouping, not a unit themselves

• 1 mole = 6.022*1023 things (Avogadro’s number)

• Moles are not conserved, because you can group things in different ways, which changes how many of them there are.

• 1 mole of protons/neutrons weighs 1 gram.

• Nature is full of terrifying animals.