- The concept of moles in chemistry
is famously difficult to explain and understand.
A mole is any amount of something.
So you might say, I have a mole of carbon dioxide
or I have a mole of glucose.
And you can see why it's a difficult concept to explain
if you just look at the definition of the mole.
So by definition, a mole is 6.02214076 times 10
to the 23 particles or things.
In other words, if you have 602 sextillion, 214 quintillion,
and 76 quadrillion things,
then you have a mole of those things.
So I guess you understand what a mole is now,
except, why is it that?
And how do I use it?
It's a really opaque definition.
We need to get to the bottom of the purpose of the mole.
And actually I think moles fall into a category of things
that are difficult to explain and understand
until you first look at the problem that that thing solves
and then it becomes really easy.
The offside rule is another example.
Maybe I'll explain the offside rule at the end
of this video as well.
But for now, we're gonna look at moles.
So by the end of this video,
you will really understand moles.
So what problem does the mole solve for chemists?
Well, chemists like reacting things together.
So maybe they've got some hydrogen over here
and some fluorine over here.
They wanna react them together to make hydrogen fluoride
but they wanna do it in such a way
that there's nothing left over.
There's no residual hydrogen, there's no residual fluorine.
How do you achieve that?
Well, you start by looking at the chemical equation
and you can see from this chemical equation here,
hydrogen plus fluorine goes to hydrogen fluoride.
By the way, this isn't a realistic chemical reaction.
This is a simplified view of the process
that is instructive for this explanation.
But anyway, you've got one of each thing
and they join together to create a molecule
that has one of each thing in it.
It's very simple.
So you need the exact same number of hydrogens
and fluorines.
How do you do that?
You could count them out.
So you could count out a trillion hydrogens
and a trillion fluorines.
You know you've got the same amount of both.
There'll be nothing left over if you react them together.
But that's impractical.
So you could do what chemists always do,
which is to weigh out the different chemicals.
So you weigh out one gram of hydrogen
and one gram of fluorine
then you've got the same amount of both,
except that doesn't work
because fluorine atoms are heavier than hydrogen atoms.
So if you've got one gram of each,
you'll have more hydrogen atoms than fluorine atoms.
So what you have to do is look up the relative masses
of hydrogen and fluorine.
And when you do that, you find that fluorine
is 19 times heavier than hydrogen.
If you were going to react single atoms together,
so you've got one atom of hydrogen, one atom of fluorine,
the ratio of the masses in that chemical reaction
would be 19 to one.
And so long as you keep that mass ratio the same,
you know you'll have the same number of hydrogens
as you do fluorines.
So for example, if you've got one gram of hydrogen,
you'll need 19 grams of fluorine.
Or you could do two grams of hydrogen
and 38 grams of fluorine, or you could do 10 ounces
of hydrogen and 190 ounces of fluorine.
So long as the ratio is one to 19,
you know you'll have the same number of atoms in each pile.
So how do we know that an atom of fluorine
is 19 times heavier than an atom of hydrogen?
While most of the massive anatomy is in the nucleus
because the electrons are so incredibly light,
so we just need to look inside the nucleus
and see what's there.
For hydrogen, the nucleus is just a proton, one proton.
So we could say that the mass of hydrogen is equivalent
to one proton or to use the proper jargon,
we talk about atomic mass units.
So we would say that hydrogen has an atomic mass of one.
I'm oversimplifying slightly which I clear up at the end,
but go with it for now.
If you look inside the nucleus of fluorine,
it has nine protons and 10 neutrons.
So it has an atomic mass of 19.
Well, so far our imaginary chemist
hasn't come up against anything particularly challenging,
that's for two reasons.
The first reason is that the chemical reaction
is really simple.
You've got two atoms going in,
you got one molecule coming out
and that molecule is made of one of each of the atoms.
The second reason is that one of the chemicals going in
is hydrogen and that has an atomic mass of one,
which makes the mathematics really easy.
In reality, chemical reactions
are usually much more complicated than that.
For example, the chemicals going in won't be atoms,
they'll be molecules.
And the results of the chemical reaction might
be more complicated.
Like it will have six of one thing and two of the other,
which makes the mathematics more complicated.
So let's imagine a slightly more complicated scenario.
This time, let's look at beryllium oxide.
Here's the chemical equation for it.
Again, this isn't how you make beryllium oxide.
It's a summary view.
It's still really simple but now the masses
are more interesting.
So if we were to react a single atom of beryllium
and a single atom of oxygen,
what would the ratio of the masses be?
Well, oxygen has an atomic mass of 16.
Beryllium has an atomic mass of nine.
So the ratio is 16 to nine
which is why I call this chemical the widescreen chemical.
(laughing)
So when you're reacting single atoms together,
the ratio of the masses is 16 to nine.
And so long as you keep that mass ratio the same,
you'll have the same number of both atoms
and you won't have anything leftover
after the chemical reaction takes place.
That's called a stoichiometric ratio, by the way.
So a sensible choice might be to have 16 grams of oxygen
and nine grams of beryllium.
This logic works for molecules as well, by the way.
So if the chemicals going into your reaction are molecules,
you just count up the number of protons and neutrons
in the molecule and that gives you the atomic mass
of the molecule.
And so chemistry proceeds in this way.
Whenever a chemist wants to react some things together,
they work out the atomic mass of those things
and they get that much of the stuff,
but in grams or kilograms or whatever,
or tens of grams or tens of kilograms.
So you can imagine chemists having conversations like these.
How much oxygen have you got there?
Well, you know, the atomic mass of oxygen,
well that but in grams.
How much hydrogen have you got there?
Well, you know the atomic mass of hydrogen, well that,
but in grams.
Well actually double that because I'm making water.
So for every oxygen, you've got two.
So that carries on for a number of years.
And then I imagine some clever chemist comes along
and says do you know what?
Instead of every time saying, you know the atomic mass of X,
well that, but in grams, why don't we just say moles?
So you say, I have a mole of sodium chloride.
I have a mole of oxygen.
And we all just agree that when you say that,
what you mean is, you know the atomic mass of that thing.
Well that, but in grams.
So for example, aluminum has an atomic mass of 27.
So if you've got 27 grams of aluminum,
you have one mole of aluminum
and it just makes everything a lot easier.
You can look at any chemical equation and just go,
so I clearly need one more of this, three moles of this
and I'll get two moles of this.
Then when it comes to actually doing the chemical reaction,
you just look up the atomic masses
of the chemicals going in.
You get that much of the chemicals in grams,
and you know you have the perfect ratio of chemicals.
You might be thinking at this point
that the definition I just gave you of the mole
is really different to the stated definition that I gave
at the start of the video with that really big number in it.
I'll get to that in a second,
but first I wanna talk about a simplification
that I made earlier.
I said that hydrogen has an atomic mass of one.
Oxygen has an atomic mass of 16.
That's not quite true.
For example, hydrogen has an atomic mass of 1.008.
Oxygen has the atomic mass of 15.999.
They will a little bit off from whole numbers.
The reason for that, well, there's two reasons.
The first is relativity.
We know from relativity that energy has mass,
equals MC squared,
and the bonds between protons and neutrons contain energy
and that energy has some mass.
The second reason is isotopes.
So hydrogen is usually one proton
with an electron going round it.
And that's the vast majority of hydrogen,
but occasionally you get hydrogen where the nucleus
is a proton and a neutron, that's called deuterium.
There's a tiny amount of that occurring naturally on earth.
So if you wanna say what the atomic mass
of naturally occurring hydrogen is,
you need to take into account that tiny amount of hydrogen
that is actually deuterium.
That's not just true for hydrogen, of course,
other atoms have different isotopes as well.
And because the atomic mass of atoms deviate slightly
from just counting the number of protons and neutrons
in the nucleus, we need to be really careful
to properly formally define atomic mass units.
So you could, for example, say, okay, by definition,
one proton on its own, not bonded to anything else
has an exact atomic mass of one.
And then everything else is based on that.
For some reason, scientists decided not to do that.
They went with carbon instead,
maybe 'cause it's easier to measure.
It's more abundant, something like that.
So the formal definition of atomic mass units
is one atomic mass unit is equal to 1/12
of the mass of carbon 12,
where carbon 12 is the isotope of carbon
that has six protons and neutrons in the nucleus.
Which is why an older definition of the mole
was the amount of atoms in 12 grams of the carbon isotope,
carbon 12,
which arguably is even more confusing.
How does that relate to the modern definition
with that huge number in it?
Well, if you define atomic mass unit as 1/12
of the mass of carbon 12,
you don't actually need to know the absolute mass
of one of those atomic mass units.
You don't need to know what it is in grams or kilograms
or anything like that.
You don't need that information.
You can carry on just comparing the atomic mass units
of different chemicals and then weigh out moles
of those things
and proceed with your chemical reactions with confidence.
But as we became better scientists,
we were able to eventually work out the absolute mass
of a proton or the absolute mass of an atomic mass unit.
And it's some tiny fraction of a gram.
And so from that point, you can work out
how many atomic mass units there are in a gram
and that's the huge number.
It's called Avogadro's number.
And so now we've come full circle.
If you measure out a gram of something
that has an atomic mass of one,
then you will have an Avogadro's number of that thing.
Generally, the use of moles is confined to chemistry,
but you could use it for other things.
For example, there is one micro mole of sand grains
on the earth.
I have a drawer in my house with half a mole
of cables in it.
I said it also explain the offside rule and I have,
it's in a separate video.
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