Name: The mole and Avogadro's number | Atomic structure and properties | AP Chemistry | Khan Academy
Uploaded: 2020-03-27T17:41:02.000Z
Duration: 8 min 57 s
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Reflexive pronouns

we introduced ourselves to the idea of average atomic mass,

Adverbs of degree

which we began to realize could be a very useful way

of thinking about a mass at an atomic level,

is connect it to the masses that we might actually see

You're very unlikely to just be dealing with one atom,

or just a few atoms, or just a few molecules.

You're more likely to deal with several grams

So, how do we go from the masses at an atomic scale

or in, I guess you could say, r-scale of the world.

Well, the chemistry community has come up with

They said, all right, let's think about a given element.

6.94 unified atomic mass units per atom, atom of lithium.

What if there were a certain number of atoms of lithium

so times certain, certain number of atoms,

then I will actually end up with 6.94 grams,

So, if you have a sample with this number of lithium atoms,

that sample is going to have a mass of 6.94 grams.

you will have a mass of that same number in terms of grams.

Now, you might be saying, is there a name for this number,

named in honor of the early 19th century Italian chemist,

And, in most contexts, because you're not normally

dealing with data with this many significant digits,

Now, there's another word that it's very useful

to familiarize yourself with in chemistry,

a mole is just saying you have this much of something.

The word mole was first used by the German chemist

Wilhelm Ostwald at the end of the 19th century,

So, if I say I have a mole of lithium atoms,

Exact same idea, it's just that Avogadro's number

is much hairier of a number than a dozen.

So, let's use our new found powers of the mole

and Avogadro's number to start doing some useful things.

Let's say that someone were to walk up to you and say,

hey, you, I have a 15.4 milligram sample of germanium.

How many atoms of germanium am I dealing with?

Pause this video and try to think about that.

the periodic table of elements was taking up.

The first step might be hey, let's convert this

And so, we can do a little bit of dimensional analysis.

And so, if you essentially multiply by one thousandth

or divide by 1,000, we're gonna get the grams of germanium.

And, you can see that in the dimensional analysis

by seeing that that is going to cancel out with that

leaving us with just the grams of germanium.

And, now that we have an expression for grams of germanium,

Well, we're going to multiply by some quantity,

and in the denominator we're going to want

grams of germanium for the dimensional analysis to work out,

grams of germanium, and in the numerator we want

the new expression to be in terms of moles of germanium.

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The mole and Avogadro's number | Atomic structure and properties | AP Chemistry | Khan Academy

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