OK, today we’re going to talk about the mole.
Now, I know what you’re thinking: ‘I know what a mole is,
it’s a small furry creature that digs holes in the ground and destroys gardens.’
And some of you might be thinking that it’s a growth on your aunt’s face with hairs sticking out of it.
Well, in this case, a mole is a concept that we use in chemistry to count molecules,
atoms, just about anything extremely small.
Have you ever wondered how many atoms there are in the universe?
Or in your body? Or even in a grain of sand?
Scientists have wanted to answer that question,
but how do you count something as small as an atom?
Well, in 1811, someone had an idea that if you had equal volumes
of gases, at the same temperature and pressure,
they would contain an equal number of particles.
His name was Lorenzo Romano Amedeo Carlo Avogadro.
I wonder how long it took him to sign autographs.
Unfortunately for Avogadro, most scientists didn’t accept the idea of the atom,
and there was no way to prove he was right.
There was no clear difference between atoms and molecules.
Most scientists looked at Avogadro’s work as purely hypothetical,
and didn’t give it much thought.
But it turned out he was right! By late 1860,
Avogadro was proven correct, and his work helped lay the foundation
for the atomic theory. Unfortunately, Avogadro died in 1856.
Now the thing is that the amount of particles in even small samples
is tremendous. For example,
If you have a balloon of any gas at zero degrees Celcius,
and at a pressure of one atmosphere, then you have precisely
six hundred and two sextillion gas particles.
That is, you have six with 23 zeros after it particles of gas in the container.
Or in scientific notation, 6.02 times 10 to the 23rd particles.
This example is a little misleading,
because gases take up a lot of space due to the high kinetic energy
of the gas particles, and it leaves you thinking atoms are bigger than they really are.
Instead, think of water molecules.
If you pour 18.01 grams of water into a glass,
which is 18.01 milliliters, which is like three and a half teaspoons of water,
you’ll have 602 sextillion molecules of water.
Since Lorenzo Romano – uh, never mind – Avogadro was the first one to come up with this idea,
scientists named the number 6.02 times 10 to the 23rd after him.
It is simply known as Avogadros’s number.
Now, back to the mole. Not that mole.
This mole. Yep, this number has a second name.
The mole. Chemists use the term mole
to refer to the quantities that are at the magnitude of 602 sextillion.
This is known as a molar quantity.
Atoms and molecules are so small, that chemists have bundled them into groups called moles.
Moles are hard for students to understand because they have a hard time
picturing the size of a mole,
or of 602 sextillion.
It’s just too big to wrap our brains around.
Remember our 18.01 milliliters of water?
Well, that’s a mole of water.
But how much is that?
Exactly what does 602 sextillion look like?
Maybe this’ll help.
Exchange the water particles for donuts.
If you had a mole of donuts, they would cover the entire earth
to a depth of eight kilometers,
which is about five miles.
You really need a lot of coffee for that.
If you had a mole of basketballs, you could create a new planet
the size of the earth.
If you received a mole of pennies on the day you were born and spent a million dollars
a second until the day you died at the age of 100, you would still have more than 99.99%
of your money in the bank.
OK. Now we sort of have an idea how large the mole is.
So how do we use it?
You might be surprised to know that chemists use it the same way
you use pounds to buy grapes, deli meat, or eggs.
When you go to the grocery store, you don’t go to the deli counter
and ask for 43 slices of salami, you buy your salami by the pound.
When you buy your eggs, you buy a dozen eggs.
When we hear the word dozen, we probably think of the number 12.
We also know that a pair is two,
a baker’s dozen is 13,
一箱(冰箱)是144，一令纸是 – 有人知道吗？
a gross is 144, and a ream of paper is – anybody?
A ream is 500.
Well, a mole is really the same thing.
For a chemist, a mole conjures up the number 6.02 times 10 to the 23rd,
not a fuzzy little animal. The only difference is
that the other quantities are more familiar to us.
So there you have it – the story of the mole,Avogadro, basketballs, and how to buy salami at the grocery store.