Hi. This is Mr. Andersen and
today I’m going to show you how to do mole conversions. Sometimes we call these mole
gram conversions. But the first thing kids get confused by is just the word mole. I know
it’s a furry animal, but the mole comes from the word molecule or the root molecule. And
so if you think of it like that, that’s an easier way to kind of get at it. Why do we
even use it? Well the reason we use mole is the mole allows you to deal with chemistry.
It allows you to deal with atoms that are really really small in the laboratory. And
so the nice thing about a mole is it gives you a usable amount. You can kind of think
of it that way. Another way to think about a mole is it’s simply a unit. And so if I
say I need a dozen eggs, you’re not scared of the word dozen. It’s not very scary. And
you shouldn’t be scared of the word mole. All it is is 6.022 times 10 to the 23rd. So
it’s a number. And so by the time you’re done with this you should be able to take the amount
of water inside this beaker, it looks like we have about 200 ml and thereby 200 grams
of water. And you should be able to figure out how many molecules of water are actually
in that. So let’s get to it. Let’s start by talking about eggs. And so a dozen is simply
the number of eggs. And so if you have a dozen eggs you know that one dozen is equal to 12
eggs. And so when you’re dealing with a mole, it’s the same way. It’s just going to be a
number of atoms or particles that you have. Also you might know that a dozen eggs weighs
24 ounces. And I didn’t know that but I learned this while making this. If you get large eggs
at the supermarket, large means that every dozen eggs weighs 24 ounces. And if you go
to extra large that’s 27 ounces. So if you’re getting Grade A Large Eggs, if you take a
dozen of those that’s going to be exactly 24 ounces. And so what we could do, you should
be able to do this in your head, is you should be able to go from eggs to dozen to ounces.
In other words if I have a dozen eggs. How many eggs do I have? Well you’d simply multiply
this times 12, when you run in this direction. And if I have one egg, how many dozen do I
have? Well you’re going to divide by 12 as you go in the other direction. And likewise
if you had a dozen eggs, weighs 24 ounces, you simply are going to take it times 24 to
figure out how many ounces you have. And thereby as you move back you’re going to divide by
24. And so if you think about this example when you start talking about moles the next
slide shouldn’t be scary at all. If I were to say to you, let’s say I have 48 ounces
of eggs, how many eggs do I have? You should be able to figure out that that’s 24 eggs.
Okay. So let’s go to the next slide and you’ll see that it’s not really going to change.
So what is a mole? A mole is simply a number. And so it’s 6.022×10^23 particles. The other
thing that you need to know is how much a mole actually weighs. To figure out the mass,
just like we said one dozen eggs weighs 24 ounces, 1 mole always weighs the formula mass.
And so if we’re dealing with H2O for example, how do I figure out how much a mole of water
weighs? You’re going to have to figure out, as I make my H correctly, you’re going to
have to figure out the molecular mass or the formula mass of that. And so let’s go forward
for a second. So if we’re on this screen right here, I have to figure out H2O and its formula
mass. To do that well, H2O has 2 Hs. So you’ve got 1 H, 2 Hs. And then 1 oxygen. And so on
the periodic table you’re going to find the mass of each of those. So here is hydrogen.
And so it has a mass of 1.01. The other hydrogen has a mass of 1.01. And then the oxygen which
is way over here has a mass of 16.00. And so if I add all of those masses up, the mass
of water is going to 18.02. And so if we want to say how much mass does one mole of water
have? It’s going to be 18.02 grams. And so you’ll have to do this if you’re ever going
from moles to grams. You’ll have to get a periodic table out. And you’re going to have
to figure out what’s the formula mass of that. And so let’s go back to this then. So just
like we could go from one egg to the dozens to the ounces of eggs we can go from particles,
and that could be either atoms or molecules to moles and then finally to grams. One mole
is going to have exactly 6.022 x10^23 particles. And so if you wanted to go from moles to particles
you’d simply multiply it times 6.022×10^23. Just like we did with a dozen eggs. And thereby
if you want to go from particles back to moles, then we’re going to have to divide. And if
we want to go from moles to grams we’re going to have to multiply it times the formula mass
or the mass of that molecule or atom. And then if we want to go back we’re going to
divide it. And so those two strategies allow you to make these kind of work. But I always
want you to understand how to do this using factor-label method. And if you don’t know
how to do that then make sure you look at the video on how to do factor-label. So let’s
do some actual problems, because that’s what you want to know how to solve. And so let’s
say we have this. Convert 0.200 moles of H2SO4 to grams. And so if I go back and figure out
where am I going to go, well right here they’re telling me I’ve got moles and I want to get
all the way to grams. And so which one of these am I going to use? We’re going to use
this first conversion. It’s where one mole is equal to the formula mass. We’re going
to use that conversion right here. So let’s go forward. So if you’re ever doing factor-label
I always tell you remember write .200 moles over 1. And now I’m just going to multiply
it times a factor. So I want to get rid of the moles so I’m going to put 1 mole on the
bottom. And then I want to get to grams is my goal. And so I’m going to get to grams
on the top. So I want to find that conversion between moles and grams. And remember 1 mole
of anything is the atomic or the formula mass of that. And so we have to figure out H2SO4.
So let’s go find that. So H2SO4 as I find my periodic table and let’s switch colors,
H2SO4 is going to have 2 hydrogens. And so there’s two of these 1.01. So that’s going
to be 2 x 1.01 which is equal to 2.02. Also in H2SO4, let’s write that down here, H2SO4,
we’ve got 1 sulfur. So I find sulfur on here. Sulfur has a mass on 32.06. And then we’ve
got to figure out, I’ve got 4 oxygens. So it’s O4. So I’m going to take 4 times oxygen,
and oxygen is 16. And so 4 times 16 is 64. So I have to add all of these up. So I’m going
to take this, 2 excuse me. This is an 8. This is a 0. This is a point. And this is 96, 98.08.
So this is the formula mass of H2SO4. And so once I’ve got that let’s go to where our
actual problem is. So we’ve got 98., let me go back. I forgot it. 98.08. So I have 98.08
grams in one mole of, this is sulfuric acid. So now I just, I’m going to multiply that.
So I’m going to cross off my moles. Get rid of my moles. And so now I’ve gotten to grams.
And so now if you look at it I’m just going to take 0.200 times 98.08. So let me grab
a calculator. So that’s going to be 0.2 times 98.08 and I get 19.616. So let me go back
to the presentation. 19.616. And so it’s 19.616 grams of H2SO4. Okay. Now let’s do significant
digits. This has how many significant digits? 3. And so this would not be the correct answer.
The correct answer would be 19.6 grams of H2SO4. So that’s going from moles to grams.
And we just simply multiplied it by the number of grams. Okay. Let’s go to a little bit harder
one. This one we’re going to take 102.8 grams of water and then were going to figure out
how many molecules that is. So if we go back to our little chart, they’re telling us now
how many grams we have. And we have to go all the way over here to particles. And so
I’m going to have to use both of these conversions. This conversion is going to get me from grams
to moles. And then this conversion is going to get me to the actual number of particles.
So let’s take a stab at that. Here we go. So first thing I’m going to do is I’m going
to write out what I start with. That’s 102.8 grams. And I’m going to put that over 1. This
is H2O. Now I’m going to multiply that, so I’m going to multiply that times, I’m going
to put grams of H2O on the bottom. Because I want to get rid of that. And I always have
to go the mole. Mole is always going to be in the middle. So I’m going to put moles on
the top. I know 1 mole, and we’ve already done the water one, H2O, we already said that
that is 18.02 grams of water. So now my grams of water would cross out. Now the next thing
I want to do is I want to go from here, from moles to molecules. So what’s my conversion
to do that? 1 mole of anything is always Avogadro’s number, so 6.022×10^23 of molecules. And so
now my moles will cross and I love factor label because you can always look through
and make sure, yep, I’m getting to molecules and that’s what the question is trying to
ask. And so if we do that on the calculator, let me switch to a calculator. So we’re going
to take, it is 102.8 and I’m going to multiply that times Avogadro’s number, which is 6.022,
oops I screwed up there. So let’s try that again. So it’s 102.8 times 6.022, I love this
key right here. What it does is allows me to write in what’s the exponent, so it’s times
10^23 equals that. And then I’m simply going to divide that by 18.02. And so the right
answer for that is 3.435 x 10^24. 3.435. So that is 3.435 x 10^24. Let’s do significant
digits. This has 4 significant digits. So that’s going to be my right answer. And so
that’s going to tell me how many molecules of water we have. So if you want to go back,
you could go back to that picture I had right at the beginning and you should be able to
figure out if this is 200.0 grams of H2O. Could you figure out how many molecules that
would be? If you were paying attention you should be able to. And I hope that’s helpful.