Mr. Andersen shows you how to convert moles to grams and moles to molecules.
Transcript Provided by YouTube:
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.
This post was previously published on YouTube.
Photo credit: Screenshot from video