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Let's talk about a concept that probably confuses

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chemistry students the most on some level.

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But on some level it's also one of the simplest concepts.

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And that's the idea of a mole, which in chemistry is

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different than the thing digging up your backyard, or

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the thing you want to get removed from your left eye.

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A mole in chemistry is just a number.

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It's just a number, and the number is 6.02

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times 10 to the 23.

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So it's a very huge number.

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And this is also called Avogadro's number.

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Maybe I will do a video on Avogadro.

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But that's all you need to know.

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A mole is just a number.

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There are kind of more Byzantine

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definitions of a mole.

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This actually is not-- actually, let me copy and

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paste it from Wikipedia.

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This is Wikipedia's definition of a mole.

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And you hopefully at the end of this video you'll see that

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they're equivalent.

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But if you're just getting exposed to the concept, this

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to me, it's just not an easy concept.

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Basically, a "a mole is defined as the amount of

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substance of a system that contains as many elemental

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entities as there are atoms in 12 grams of carbon 12." Well,

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I just told you that a mole is 6.02 times 10 to the 23.

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So if you just take the last part, atoms in 12

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grams of carbon 12.

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So that means that there are 1 mole of carbon 12-- let me

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write it like that-- carbon 12.

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There are 1 mole of carbon 12 atoms in 12 grams of carbon.

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And so that's why a mole is useful.

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So I could have just instead of writing 1 mole, I could

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have replaced this as there's 6.02 times 10 to the 23 carbon

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atoms, carbon 12 atoms in 12 grams of carbon.

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How do you figure that out?

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Or I guess, what else does this mean?

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I mean, we just added in carbon, they said it's the

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amount of substance of any molecule, if you convert

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between atomic mass units and grams.

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This I find very confusing.

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How can we apply this in other places?

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So the first thing to realize is a mole is just a way of

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translating between grams and atomic mass units.

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One carbon 12 atom is what?

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What's its mass number?

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It's 12.

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That's why it's called carbon 12 instead of carbon 14.

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So its mass is 12 atomic mass units.

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So if you have something that has a mass of 12 atomic mass

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units and you have a mole of them, or you have 6.02 times

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10 to the twenty three of them, all of those atoms

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combined will have a mass of 12 grams.

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So another way to think about it is 1 gram is equal to 1

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mole of atomic mass units.

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I'll write amu's like that.

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Or you can write 1 gram is equal to 6.02 times 10 to the

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23 atomic mass units.

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And the reason why this is useful-- and it's kind of

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addressed in this Wikipedia definition there-- is it helps

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us translate between the atomic world-- where we deal

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with atomic mass units and we deal with, oh, we've got an

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extra neutron now, let's add one to our atomic mass

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number-- and translating between that atomic world and

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our everyday world where we deal in grams.

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And just so you know, a gram is still a pretty

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small amount of mass.

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It's 1/1,000 of a kilogram.

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A kilogram is about 2 pounds.

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So this is about 1/500 of a pound.

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So this is not much.

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So there's a ton of atoms in a very small amount of-- in 1

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gram of carbon, or at least in 12 grams of carbon, you have a

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ton of atoms. You have 6.02 times 10 to the 23.

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And just to hit the point home, I probably should have

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talked about this in the atom.

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This is a huge number.

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To maybe visualize it, if you think of-- I was told that in

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the diameter of a hair, if this is a hair and this is

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diameter of the hair, if you go this way there 1 million

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carbon atoms. 1 million carbon atoms that way.

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Or if you were to take an apple and you were to try to

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figure out what fraction, if you were to make one of the

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atoms of an apple-- and obviously, an apple has a

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bunch of different types of atoms in it-- but if you were

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to take one of the atoms and make it the size of the apple,

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then the apple would be the size of the earth.

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So an apple atom is to an apple as an

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apple is to the earth.

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So these are obviously-- it's hard for us to even process

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things of this size.

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When you just have one gram of-- well, let's say you have

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1 gram of hydrogen.

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1 gram of hydrogen.

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If you have 1 gram of hydrogen, that means you have

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1 mole of hydrogen.

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How do I know that?

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Because hydrogen's atomic mass number is 1.

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So in general, if you just take any element-- so what is

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the mass of, let me just pick, 1 mole of aluminum?

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So if I were to take 6.02 times 10 to the 23 aluminum

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atoms, what is the mass of that collection?

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Well, each of them have an atomic mass number of 13.

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So it's 13 amu's-- I don't have to put the s there--

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times six point-- well, I won't

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write that way, actually.

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That'll probably just confuse you.

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The easy way to think about is if you have a mole of an atom,

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you take its mass-- I was taking its atomic number,

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that's not good-- you take its mass number.

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In this case let's say it's 27.

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So we're dealing with aluminum 27.

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You take its mass number, and if you have 1 mole of it, then

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the mass of that will be 27 grams.

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So that literally, when you have one mole of an atom it's

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a direct translation between its mass number and grams. 1

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mole of iron, let's say iron 56-- there's obviously many

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isotopes or iron-- let's say we're dealing with iron 56.

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You normally don't hear it like that, but let's say we're

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dealing with the isotope of iron that has a

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mass number of 56.

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So if I have 1 mole of this, 1 mole of this atom right here,

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that's going to have a mass of-- the math isn't difficult

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here-- 56 grams. And if you think about it, how many

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atomic mass units is this?

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Well, this is 56 atomic mass units per atom.

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Then you have a mole of those, so you have 6.02 times 10 to

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the 23 times 56 atomic mass units.

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And then you divide it by the number of atomic

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mass units per gram.

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And you end up with 56 grams.

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But the easy way to think about it is you just take

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whatever the mass number is.

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If you have silicon, if you have a mole of silicon, a mole

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of silicon will have a mass-- I don't want to say weight

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because this should apply to any planet-- of 28 grams.

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What about 2 moles of silicon?

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And I'll write its mass number.

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Let's say silicon has a mass number of 28.

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Two moles of silicon.

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Well, 1 more would have a mass of 28 grams, so 2 moles is

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going to have a mass of 56 grams.

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If I were to say, let's say I had 4 moles of oxygen, which

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has a mass number of 16.

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What is the mass of that?

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This is a huge number of oxygen atoms-- what would be

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the mass of that?

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Well, it would be 4 times-- 1 mole of oxygen would have a

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mass of 16 grams, so 4 moles has 64 grams.

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It's confusing because we're not used to using a word like

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moles as a number, but all it is is a number.

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And the easy way to think about is that it lets us

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translate between this atomic mass unit number and grams.

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And you say, well, how do I get that many grams?

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Well, I have to have 6.02 times 10 to the 23 carbon

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atoms for that collection of carbon to have a mass of 12

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grams. That's all that mole means.

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It's just a number.

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And I encourage you to kind of play around with a lot of what

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we talked about.

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Because it's super important to have the intuition behind

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moles, otherwise you'll get confused later on when we

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start getting into energies in terms of it requires

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kilojoules per mole, and what is the energy of this reaction

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and all that type of stuff.

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So just really try to make sure you digest this as well

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as possible.

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And let me know if you don't and I'll maybe make another

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video on this because it's so important.

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