Lab Values and Concentrations
The Mole and Avogadro's Number Introduction to the idea of a mole as a number (vs. an animal)
The Mole and Avogadro's Number
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- Let's talk about a concept that probably confuses
- chemistry students the most on some level.
- But on some level it's also one of the simplest concepts.
- And that's the idea of a mole, which in chemistry is
- different than the thing digging up your backyard, or
- the thing you want to get removed from your left eye.
- A mole in chemistry is just a number.
- It's just a number, and the number is 6.02
- times 10 to the 23.
- So it's a very huge number.
- And this is also called Avogadro's number.
- Maybe I will do a video on Avogadro.
- But that's all you need to know.
- A mole is just a number.
- There are kind of more Byzantine
- definitions of a mole.
- This actually is not-- actually, let me copy and
- paste it from Wikipedia.
- This is Wikipedia's definition of a mole.
- And you hopefully at the end of this video you'll see that
- they're equivalent.
- But if you're just getting exposed to the concept, this
- to me, it's just not an easy concept.
- Basically, a "a mole is defined as the amount of
- substance of a system that contains as many elemental
- entities as there are atoms in 12 grams of carbon 12." Well,
- I just told you that a mole is 6.02 times 10 to the 23.
- So if you just take the last part, atoms in 12
- grams of carbon 12.
- So that means that there are 1 mole of carbon 12-- let me
- write it like that-- carbon 12.
- There are 1 mole of carbon 12 atoms in 12 grams of carbon.
- And so that's why a mole is useful.
- So I could have just instead of writing 1 mole, I could
- have replaced this as there's 6.02 times 10 to the 23 carbon
- atoms, carbon 12 atoms in 12 grams of carbon.
- How do you figure that out?
- Or I guess, what else does this mean?
- I mean, we just added in carbon, they said it's the
- amount of substance of any molecule, if you convert
- between atomic mass units and grams.
- This I find very confusing.
- How can we apply this in other places?
- So the first thing to realize is a mole is just a way of
- translating between grams and atomic mass units.
- One carbon 12 atom is what?
- What's its mass number?
- It's 12.
- That's why it's called carbon 12 instead of carbon 14.
- So its mass is 12 atomic mass units.
- So if you have something that has a mass of 12 atomic mass
- units and you have a mole of them, or you have 6.02 times
- 10 to the twenty three of them, all of those atoms
- combined will have a mass of 12 grams.
- So another way to think about it is 1 gram is equal to 1
- mole of atomic mass units.
- I'll write amu's like that.
- Or you can write 1 gram is equal to 6.02 times 10 to the
- 23 atomic mass units.
- And the reason why this is useful-- and it's kind of
- addressed in this Wikipedia definition there-- is it helps
- us translate between the atomic world-- where we deal
- with atomic mass units and we deal with, oh, we've got an
- extra neutron now, let's add one to our atomic mass
- number-- and translating between that atomic world and
- our everyday world where we deal in grams.
- And just so you know, a gram is still a pretty
- small amount of mass.
- It's 1/1,000 of a kilogram.
- A kilogram is about 2 pounds.
- So this is about 1/500 of a pound.
- So this is not much.
- So there's a ton of atoms in a very small amount of-- in 1
- gram of carbon, or at least in 12 grams of carbon, you have a
- ton of atoms. You have 6.02 times 10 to the 23.
- And just to hit the point home, I probably should have
- talked about this in the atom.
- This is a huge number.
- To maybe visualize it, if you think of-- I was told that in
- the diameter of a hair, if this is a hair and this is
- diameter of the hair, if you go this way there 1 million
- carbon atoms. 1 million carbon atoms that way.
- Or if you were to take an apple and you were to try to
- figure out what fraction, if you were to make one of the
- atoms of an apple-- and obviously, an apple has a
- bunch of different types of atoms in it-- but if you were
- to take one of the atoms and make it the size of the apple,
- then the apple would be the size of the earth.
- So an apple atom is to an apple as an
- apple is to the earth.
- So these are obviously-- it's hard for us to even process
- things of this size.
- When you just have one gram of-- well, let's say you have
- 1 gram of hydrogen.
- 1 gram of hydrogen.
- If you have 1 gram of hydrogen, that means you have
- 1 mole of hydrogen.
- How do I know that?
- Because hydrogen's atomic mass number is 1.
- So in general, if you just take any element-- so what is
- the mass of, let me just pick, 1 mole of aluminum?
- So if I were to take 6.02 times 10 to the 23 aluminum
- atoms, what is the mass of that collection?
- Well, each of them have an atomic mass number of 13.
- So it's 13 amu's-- I don't have to put the s there--
- times six point-- well, I won't
- write that way, actually.
- That'll probably just confuse you.
- The easy way to think about is if you have a mole of an atom,
- you take its mass-- I was taking its atomic number,
- that's not good-- you take its mass number.
- In this case let's say it's 27.
- So we're dealing with aluminum 27.
- You take its mass number, and if you have 1 mole of it, then
- the mass of that will be 27 grams.
- So that literally, when you have one mole of an atom it's
- a direct translation between its mass number and grams. 1
- mole of iron, let's say iron 56-- there's obviously many
- isotopes or iron-- let's say we're dealing with iron 56.
- You normally don't hear it like that, but let's say we're
- dealing with the isotope of iron that has a
- mass number of 56.
- So if I have 1 mole of this, 1 mole of this atom right here,
- that's going to have a mass of-- the math isn't difficult
- here-- 56 grams. And if you think about it, how many
- atomic mass units is this?
- Well, this is 56 atomic mass units per atom.
- Then you have a mole of those, so you have 6.02 times 10 to
- the 23 times 56 atomic mass units.
- And then you divide it by the number of atomic
- mass units per gram.
- And you end up with 56 grams.
- But the easy way to think about it is you just take
- whatever the mass number is.
- If you have silicon, if you have a mole of silicon, a mole
- of silicon will have a mass-- I don't want to say weight
- because this should apply to any planet-- of 28 grams.
- What about 2 moles of silicon?
- And I'll write its mass number.
- Let's say silicon has a mass number of 28.
- Two moles of silicon.
- Well, 1 more would have a mass of 28 grams, so 2 moles is
- going to have a mass of 56 grams.
- If I were to say, let's say I had 4 moles of oxygen, which
- has a mass number of 16.
- What is the mass of that?
- This is a huge number of oxygen atoms-- what would be
- the mass of that?
- Well, it would be 4 times-- 1 mole of oxygen would have a
- mass of 16 grams, so 4 moles has 64 grams.
- It's confusing because we're not used to using a word like
- moles as a number, but all it is is a number.
- And the easy way to think about is that it lets us
- translate between this atomic mass unit number and grams.
- And you say, well, how do I get that many grams?
- Well, I have to have 6.02 times 10 to the 23 carbon
- atoms for that collection of carbon to have a mass of 12
- grams. That's all that mole means.
- It's just a number.
- And I encourage you to kind of play around with a lot of what
- we talked about.
- Because it's super important to have the intuition behind
- moles, otherwise you'll get confused later on when we
- start getting into energies in terms of it requires
- kilojoules per mole, and what is the energy of this reaction
- and all that type of stuff.
- So just really try to make sure you digest this as well
- as possible.
- And let me know if you don't and I'll maybe make another
- video on this because it's so important.
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