In this video I want you to think about the mole as an amount of substance.
In fact, it's one of the fundamental units for amount of substance.
You've probably heard of one mole equals 6.02 x 10^23 particles where particles mean molecules or atoms.
But the mole can also tell you about the mass of a substance in grams or
even the volume of a gas in liters.
Will use the mole map here to develop and guide our thinking as we learn how to do mole conversions.
Let's jump right in and try one.
Here we're given 3.5 moles of H2 gas and we're going to convert to liters.
So when we're dealing with gases, we're dealing with liters and it doesn't matter that it's H2. It could be another gas. It'll still be the same calculation.
We're going from moles to liters,
so we multiply by 22.4.
We have 3.5 moles
and we multiply by 22.4 and the units are liters per mole. We'll worry about those in a bit.
We do the math 3.5 x 22.4. That's 78.4.
Moles cancel out so we're left with liters.
So to go from moles to liters we multiply it by 22.4.
If we had liters and we wanted to go to moles we would divide by 22.4.
Okay, now you try one.
Okay, so now you're a pro at liters to mole conversions.
So for the rest of the video we'll look at moles to grams,
probably the most common conversion. Moles to particles. And then multi-step problems where you have to go through several conversions to get your final answer.
But first I want to talk just a bit more about the mole map.
The first thing to notice is the mole is at the center. If we want to go from a unit like grams to liters,
we will have to go through the mole.
The second thing is whenever we convert from a mole to another unit we multiply.
You might even say we mole-tiply. Sorry.
One final thing to note about the mole map. Each conversion factor has units and we need to respect those as we solve problems.
The good thing is they'll serve as a way to check to make sure we set the problem up right and our answer is accurate.
All right! On to moles to grams and grams to moles conversions.
When we're dealing with grams, we're dealing with mass. And we're dealing with the mass of these different elements on the periodic table.
To convert from moles to grams and grams to moles we have to figure out the molar mass.
The periodic table provides a list of the molar mass for each element.
So it's really convenient. That number is usually found right below the element symbol.
So if we wanted to know the mass of one mole of sodium would go to the periodic table find sodium.
It's 22.99 and the units are grams per mole.
Pause and find the molar mass for calcium.
For calcium one mole equals 40.08 grams. So it's molar mass is 40.08 grams per mole.
So if we had two moles of calcium, how many grams would that be?
Well one moles 40.08, so two will just be double: 80.16 grams.
Pause and convert the given number of moles to grams.
The last one with H2, that's a bit of a trick. Since we have two hydrogen atoms
we need to add the molar mass for each hydrogen together. That would give us the mass of one mole of H2...
and then we multiply it by the two since we have two moles.
So this brings up molecules are compounds. How do we find the molar mass for something like H2O?
To find the molar mass for H2O we just go to the periodic table and we look up each element.
We find its molar mass and then add them together. So hydrogen's 1.01 grams per mole.
We have these two hydrogens, so let's multiply that by two.
Plus oxygen that's 16.00 grams per mole.
We add them up and we get 18.02 grams per mole.
That's the molar mass for H2O. And it just says that one mole weighs 18.02 grams.
All right grams and moles. If you can find the molar mass, you can probably already do these calculations just using the map here.
Give it a try pause and solve this one.
So I have 4.00 moles
and I want to get that to grams. So I have my moles and
going from moles to grams I mole-tiply.
So I'll multiply my 4.00 moles times the molar mass of H2O.
We just found that to be 18.02 g/mol.
And when I do the math I get 72.08.
Moles, those are going to cancel out and the units will be grams.
And we're done.
Some people like to use a formula to convert from grams to moles and moles to grams. So you could use this formula.
Let's try it out. We have moles equals grams over a molar mass. So we have moles, our grams
3.72 grams.
And we'll divide that by the molar mass of HCl.
So I look up H and Cl in the periodic table add them together. I get 36.46
and that's grams per mole. My grams those will cancel out and I'll be left with moles.
And when I do the math I get 0.10 moles.
Let's look at one last way to convert from moles to grams.
Building upon what we've learned and we'll do these conversions the way scientists do.
So a can of Dr. soda
weighs about 400 grams, and of that 350 grams or water.
We could convert this 350 grams to moles by dividing by the GFM (molar mass) of water that would give us about
19 moles of water. In just a bit you'll learn that you can convert moles to molecules.
19 moles is about 1 x 10^25 molecules.
The point is that in reality nothing's happened to our can of Dr. B.
It sits here unopened, beckoning. What has changed are the units. We have moles grams and molecules.
That's useful because in the lab
I might want to weigh this can and I'd have to use grams as my units to weigh.
If I was looking at a balanced chemical equation and the coefficients I'd want to think about moles.
That would be more useful to me. And sometimes you want to think about molecules.
Again, reality has not changed. Here's our can of soda. What has changed are the units.
That means all of these numbers are the same thing. They're equal.
So we have 3.72 grams of HCl, we want to go from grams to moles. We know that one mole
of HCl, we can look that up on the periodic table is 36.46 grams. These two numbers:
they're equal.
Just that they have different units. We can use this to find conversion factors.
Since one mole of HCl equals 36.46 grams, we could say 36.46 grams
divided by one mole of HCl ...
well, since they're the same thing that would just equal one. And that would be one of our conversion factors. In fact, grams per mole,
that's molar mass. So molar mass, that's a conversion factor. Let's do one more. We could flip this and put moles on top.
Since these are the same thing they equal one as well. So we're kind of doing the same thing
we've been doing all along with molar mass and moles and grams, but let's set this up using conversion factors.
I have 3.72 grams, and I want to change it to moles.
So I want my grams to cancel out and I want to be left with moles.
So I'm going to choose the conversion factor from one of these it cancels grams out.
To do that I'm going to need grams to be on the bottom. So I'm going to multiply this by
one mole of HCl
is that thirty six point four six grams.
When I do the math
I multiply the top by the one mole of HCl and now I'll divide by the bottom. Grams will cancel out ...
I'll end up with 0.10 moles,
just like before. It might seem like a lot more work
but you can use this method in all of the sciences: physic, biology, earth science ...
Converting between units becomes very easy when you understand these conversion factors. So try this problem.
Here's how I did it, double speed.
You'll see how we use these black lines to separate and set the problem up.
Successful students and scientists, that's how they do. It makes you look real smart.
Let's shift over to moles and molecules conversions and then we'll have some practice problems at the end of the video.
We can use the mole map or the conversion factors to convert from moles to molecules and molecules to moles.
Let's use the conversion factors. If you like you can use the mole map. We'll get the same answer either way
So we have one mole
equals
Avogadro's number of molecules.
That's where we'll get our conversion factors from. So we take our two moles of CH4 and
when we convert from moles to molecules, it doesn't matter that it's CH4, CO2, or NaCl.
We still use Avogadro's number. So we have two moles and
we'll put our bars across and we want to cancel moles out and end up with molecules.
We'll need moles on the bottom. One mole, that's Avogadro's number of molecules.
We multiply two by Avogadro's number and we divide by moles. So moles those will cancel out. We'll be left with molecules.
And 2.00 times Avogadro's number, that equals 12.03 x 10^23 molecules.
So that's not too bad and you can see where conversion factors can be really useful.
If you use the mall map you went from moles to molecules,
you multiplied two times Avogadro's number here and you got the same answer.
Now you try one.
Whether you use the mole map or conversion factors
you should have ended up with 18.06 x 10^23 molecules. You could change this to 1.8 x 10^24
if you wanted to have correct scientific notation. So one last type of problem.
This is a multi-step problem.
Sometimes you're asked to convert from grams to molecules, but you can't get from grams right to molecules.
And that's where the mole map really helps out so we can use the mole map to set our strategy up.
We know we have grams and we want to go to.
Molecules are particles so we can go from grams to moles by dividing by the molar mass and then from moles
we can multiply by Avogadro's number to get particles.
This highlights the centrality of the mole as a concept in chemistry.
So let's set it up with conversion factors, but you can use the mole map to solve it as well.
So whether you use the mole map to convert from grams to moles to molecules or you set the conversion factors up,
canceled your units, either way 2.00 grams of H2 is 5.96 x 10^23 molecules.
If you made it this far and you understand you're on your way to becoming a very successful chemistry student.
The key now is to practice use the mole map as your guide your mental map to solve stoichiometry
problems and then use conversion factors to make sure you set them up and have the correct units and answers.
At the end of this video you'll find some practice problems in solutions. That's a very good place to start.
This is Dr. B. and thanks
for watching.