Transcript Powerpoint 5.1 - Triton chemistry

The Mole
Not this kind
Or this kind
But First, An Analogy
If 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 Mole
A 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
Just do it as unit conversion, where 1 mole = 6.022*1023 is your conversion factor.
Example: If I have 4.35 moles of sodium, how many atoms is that?
4.35 moles Na * 6.022*1023 atoms Na
1 mole Na
= 2.62 * 1024 atoms Na
Note: see how I was careful to include moles of Na, and atoms of Na? That is a
good habit—it will be relevant later.
Example 2: If I have 8.52*1027 atoms of silicon, how many moles of silicon do I
have?
8.52*1027 atoms Si *
1 mole Si
6.022*1023 atoms Si
= 14100 moles Si
Yes, you can have decimals
There’s no reason you can’t have a half dozen bagels, right?
Same thing with moles:
1.81*1023 atoms Si *
1 mole Si
6.022*1023 atoms Si
= 0.301 moles Si
Moles Are Not Conserved
Let’s say that I have 1 mole of CO2 molecules.
Therefore, I have 6.022*1023 CO2 molecules.
If each of those is made of one carbon and two oxygens, then I have:
6.022*1023 carbon atoms and
12.044*1023 oxygen atoms
1 mole carbon atoms
2 moles of oxygen atoms
Which is
and
So how can 1 mole of atoms with 2 moles
of atoms make 1 mole?
Moles Are Not Conserved
Because 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.