Mole Conversions


Mr. Andersen shows you how to convert moles to grams and moles to molecules.

Transcript Provided by YouTube:

00:05
Hi. This is Mr. Andersen and today I’m going to show you how to do mole
00:09
conversions. Sometimes we call these mole gram conversions. But the first thing kids
00:15
get confused by is just the word mole. I know it’s a furry animal, but the mole comes from
00:21
the word molecule or the root molecule. And so if you think of it like that, that’s an
00:25
easier way to kind of get at it. Why do we even use it? Well the reason we use mole is
00:31
the mole allows you to deal with chemistry. It allows you to deal with atoms that are
00:36
really really small in the laboratory. And so the nice thing about a mole is it gives
00:40
you a usable amount. You can kind of think of it that way. Another way to think about
00:44
a mole is it’s simply a unit. And so if I say I need a dozen eggs, you’re not scared
00:48
of the word dozen. It’s not very scary. And you shouldn’t be scared of the word mole.
00:52
All it is is 6.022 times 10 to the 23rd. So it’s a number. And so by the time you’re done
00:58
with this you should be able to take the amount of water inside this beaker, it looks like
01:02
we have about 200 ml and thereby 200 grams of water. And you should be able to figure
01:08
out how many molecules of water are actually in that. So let’s get to it. Let’s start by
01:13
talking about eggs. And so a dozen is simply the number of eggs. And so if you have a dozen
01:18
eggs you know that one dozen is equal to 12 eggs. And so when you’re dealing with a mole,
01:24
it’s the same way. It’s just going to be a number of atoms or particles that you have.
01:29
Also you might know that a dozen eggs weighs 24 ounces. And I didn’t know that but I learned
01:34
this while making this. If you get large eggs at the supermarket, large means that every
01:40
dozen eggs weighs 24 ounces. And if you go to extra large that’s 27 ounces. So if you’re
01:45
getting Grade A Large Eggs, if you take a dozen of those that’s going to be exactly
01:50
24 ounces. And so what we could do, you should be able to do this in your head, is you should
01:55
be able to go from eggs to dozen to ounces. In other words if I have a dozen eggs. How
02:01
many eggs do I have? Well you’d simply multiply this times 12, when you run in this direction.
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And if I have one egg, how many dozen do I have? Well you’re going to divide by 12 as
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you go in the other direction. And likewise if you had a dozen eggs, weighs 24 ounces,
02:16
you simply are going to take it times 24 to figure out how many ounces you have. And thereby
02:20
as you move back you’re going to divide by 24. And so if you think about this example
02:26
when you start talking about moles the next slide shouldn’t be scary at all. If I were
02:30
to say to you, let’s say I have 48 ounces of eggs, how many eggs do I have? You should
02:38
be able to figure out that that’s 24 eggs. Okay. So let’s go to the next slide and you’ll
02:41
see that it’s not really going to change. So what is a mole? A mole is simply a number.
02:47
And so it’s 6.022×10^23 particles. The other thing that you need to know is how much a
02:54
mole actually weighs. To figure out the mass, just like we said one dozen eggs weighs 24
02:59
ounces, 1 mole always weighs the formula mass. And so if we’re dealing with H2O for example,
03:06
how do I figure out how much a mole of water weighs? You’re going to have to figure out,
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as I make my H correctly, you’re going to have to figure out the molecular mass or the
03:16
formula mass of that. And so let’s go forward for a second. So if we’re on this screen right
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here, I have to figure out H2O and its formula mass. To do that well, H2O has 2 Hs. So you’ve
03:28
got 1 H, 2 Hs. And then 1 oxygen. And so on the periodic table you’re going to find the
03:33
mass of each of those. So here is hydrogen. And so it has a mass of 1.01. The other hydrogen
03:40
has a mass of 1.01. And then the oxygen which is way over here has a mass of 16.00. And
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so if I add all of those masses up, the mass of water is going to 18.02. And so if we want
03:56
to say how much mass does one mole of water have? It’s going to be 18.02 grams. And so
04:03
you’ll have to do this if you’re ever going from moles to grams. You’ll have to get a
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periodic table out. And you’re going to have to figure out what’s the formula mass of that.
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And so let’s go back to this then. So just like we could go from one egg to the dozens
04:17
to the ounces of eggs we can go from particles, and that could be either atoms or molecules
04:22
to moles and then finally to grams. One mole is going to have exactly 6.022 x10^23 particles.
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And so if you wanted to go from moles to particles you’d simply multiply it times 6.022×10^23.
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Just like we did with a dozen eggs. And thereby if you want to go from particles back to moles,
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then we’re going to have to divide. And if we want to go from moles to grams we’re going
04:50
to have to multiply it times the formula mass or the mass of that molecule or atom. And
04:57
then if we want to go back we’re going to divide it. And so those two strategies allow
05:01
you to make these kind of work. But I always want you to understand how to do this using
05:07
factor-label method. And if you don’t know how to do that then make sure you look at
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the video on how to do factor-label. So let’s do some actual problems, because that’s what
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you want to know how to solve. And so let’s say we have this. Convert 0.200 moles of H2SO4
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to grams. And so if I go back and figure out where am I going to go, well right here they’re
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telling me I’ve got moles and I want to get all the way to grams. And so which one of
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these am I going to use? We’re going to use this first conversion. It’s where one mole
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is equal to the formula mass. We’re going to use that conversion right here. So let’s
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go forward. So if you’re ever doing factor-label I always tell you remember write .200 moles
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over 1. And now I’m just going to multiply it times a factor. So I want to get rid of
05:52
the moles so I’m going to put 1 mole on the bottom. And then I want to get to grams is
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my goal. And so I’m going to get to grams on the top. So I want to find that conversion
06:02
between moles and grams. And remember 1 mole of anything is the atomic or the formula mass
06:08
of that. And so we have to figure out H2SO4. So let’s go find that. So H2SO4 as I find
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my periodic table and let’s switch colors, H2SO4 is going to have 2 hydrogens. And so
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there’s two of these 1.01. So that’s going to be 2 x 1.01 which is equal to 2.02. Also
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in H2SO4, let’s write that down here, H2SO4, we’ve got 1 sulfur. So I find sulfur on here.
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Sulfur has a mass on 32.06. And then we’ve got to figure out, I’ve got 4 oxygens. So
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it’s O4. So I’m going to take 4 times oxygen, and oxygen is 16. And so 4 times 16 is 64.
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So I have to add all of these up. So I’m going to take this, 2 excuse me. This is an 8. This
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is a 0. This is a point. And this is 96, 98.08. So this is the formula mass of H2SO4. And
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so once I’ve got that let’s go to where our actual problem is. So we’ve got 98., let me
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go back. I forgot it. 98.08. So I have 98.08 grams in one mole of, this is sulfuric acid.
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So now I just, I’m going to multiply that. So I’m going to cross off my moles. Get rid
07:40
of my moles. And so now I’ve gotten to grams. And so now if you look at it I’m just going
07:44
to take 0.200 times 98.08. So let me grab a calculator. So that’s going to be 0.2 times
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98.08 and I get 19.616. So let me go back to the presentation. 19.616. And so it’s 19.616
08:19
grams of H2SO4. Okay. Now let’s do significant digits. This has how many significant digits?
08:26
3. And so this would not be the correct answer. The correct answer would be 19.6 grams of
08:32
H2SO4. So that’s going from moles to grams. And we just simply multiplied it by the number
08:39
of grams. Okay. Let’s go to a little bit harder one. This one we’re going to take 102.8 grams
08:46
of water and then were going to figure out how many molecules that is. So if we go back
08:50
to our little chart, they’re telling us now how many grams we have. And we have to go
08:56
all the way over here to particles. And so I’m going to have to use both of these conversions.
09:01
This conversion is going to get me from grams to moles. And then this conversion is going
09:05
to get me to the actual number of particles. So let’s take a stab at that. Here we go.
09:11
So first thing I’m going to do is I’m going to write out what I start with. That’s 102.8
09:15
grams. And I’m going to put that over 1. This is H2O. Now I’m going to multiply that, so
09:24
I’m going to multiply that times, I’m going to put grams of H2O on the bottom. Because
09:28
I want to get rid of that. And I always have to go the mole. Mole is always going to be
09:32
in the middle. So I’m going to put moles on the top. I know 1 mole, and we’ve already
09:36
done the water one, H2O, we already said that that is 18.02 grams of water. So now my grams
09:45
of water would cross out. Now the next thing I want to do is I want to go from here, from
09:50
moles to molecules. So what’s my conversion to do that? 1 mole of anything is always Avogadro’s
10:00
number, so 6.022×10^23 of molecules. And so now my moles will cross and I love factor
10:08
label because you can always look through and make sure, yep, I’m getting to molecules
10:11
and that’s what the question is trying to ask. And so if we do that on the calculator,
10:16
let me switch to a calculator. So we’re going to take, it is 102.8 and I’m going to multiply
10:32
that times Avogadro’s number, which is 6.022, oops I screwed up there. So let’s try that
10:40
again. So it’s 102.8 times 6.022, I love this key right here. What it does is allows me
10:51
to write in what’s the exponent, so it’s times 10^23 equals that. And then I’m simply going
10:58
to divide that by 18.02. And so the right answer for that is 3.435 x 10^24. 3.435. So
11:12
that is 3.435 x 10^24. Let’s do significant digits. This has 4 significant digits. So
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that’s going to be my right answer. And so that’s going to tell me how many molecules
11:29
of water we have. So if you want to go back, you could go back to that picture I had right
11:34
at the beginning and you should be able to figure out if this is 200.0 grams of H2O.
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Could you figure out how many molecules that would be? If you were paying attention you
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should be able to. And I hope that’s helpful.


This post was previously published on YouTube.

Photo credit: Screenshot from video