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What is a mole? The mole is simply a counting unit. We'll then, What is a counting unit?
You actually know a lot of counting units, but you probably haven't used the term counting
unit. Here's an example of a counting unit that you already know.
**Show gloves**
I have two gloves.
What word can I use in place of the number two?
I have blank gloves.
Pause
That's right, I can say that I have a PAIR of gloves. The word PAIR is a counting unit.
Let's try another one.
**show eggs**
I have 12 eggs.
What counting unit can I use instead of the number 12?
Pause
I have blank eggs?
Pause
That's right, I have a dozen eggs. The word DOZEN is a counting unit.
A counting unit is just a word that means a specific number.
So back to the mole. The mole is simply a counting unit, it refers to a specific number,
But what number?
It refers to this number.
Pause
That's a huge number! It's so large in fact, that we should use
scientific notation to write the number. Here is the number for a mole in scientific notation.
A mole is 6.022 times ten to the power of 23.
Pause
The mole is a such a large number that it's difficult to comprehend. Here's a challenge
for you.
**get bill**
I'm holding a one hundred dollar bill. If I were to place a mole of one hundred dollar
bills end to end, how far would the mole of bills reach?
Pause
Let me be clear in what I'm asking.
** get strip of bills **
Here I have a DOZEN one hundred dollar bills placed end to end.
What if I had a MOLE of one hundred dollar bills placed end to end, how far would they
go?
Pause
Maybe from New York to Los Angeles?
Longer than that. Think again, how long would one mole of bills reach?
Maybe to the moon?
Farther than that.
It turns out that one mole of bills would stretch from one end of our galaxy to the
other.
It's hard to wrap your mind around how big of a number a mole represents.
Except in this case.
(Hold carbon in hand).
I have a mole of something in my hand. Can you guess what it is that I'm holding?
Pause
Here's a hint, it's not very heavy, in fact it's only 12 grams.
Pause
If you said atoms, you're correct.
If you said carbon atoms, then you're amazing because I do have a mole of carbon atoms in
this jar.
Why do you think a chemist uses a mole, 6.022 times 10 to the power of 23 as their counting
unit?
Pause
Because atoms are really small.
In this case, a mole of carbon atoms fits nicely in my hand.
But where did that strange number, 6.022 times ten raised to the power of 23, come from?
I mean, why didn't scientists round the number to something easier to remember, like 1 times
10 raised to the power of 23?
Pause
Well the answer is in my hand.
It turns out that I'm holding the definition of a mole in my hand.
This jar contains a pure form of carbon with exactly 6 neutrons and 6 protons in its nucleus.
In other words, this is carbon-12.
And this jar is holding exactly 12 grams of this special type of carbon.
Why would I do that? Because the current definition of a mole is the number of carbon atoms that
you have IF you have 12 grams of carbon-12. That's exactly what I'm holding in my hand.
You see, if we were to painstakingly count EVERY atom in this 12gram sample, we would
have 6.022 times 10 to the power of 23 atoms.
Who decided that number of atoms in this jar would be the definition of the mole?
The answer is the International Bureau of Weights and Measures, which is an international
standards organization. The mole, abbreviated mol for those who don't want to write out
the whole word, is that "e" really that tough? is an official SI unit.
Pause
Now before you go, I want to let you know that the mole is sometimes called Avogadro's
number in honor of an Italian physicist, Amedeo Avogadro. So if someone refers to Avogadro's
number they are referring to the same number as a mole, 6.022 times ten to the power of
23.
But I prefer to say that I have a mole. It's just easier to say, no offense Avogadro.