# Mole Ratio Practice Problems

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Okay so now I'm going to do whole bunch mole ratio problems.
A whole bunch so that you can really get a handle on these. They're super important for stoichiometry;
for understanding stoichiometry. Okay, each problem I'm going to do
two ways: first I'm going to treat the equation,
the chemical equation, kind of like, kind of like a recipe.
All right, where we've got our ingredients and then we've got the stuff we're making.
This method really makes sense, like you'll understand what you're doing
but it requires a little bit of thought. Then
I'll solve each problem using conversion factor method.
The conversion factor method doesn't require any thought,
but it doesn't make any sense. So it's really easy
to get in the habit only using a conversion factor, running through the mat
but having absolutely no idea
hat you're actually doing, or why you're doing it. Okay so that's why I'm going to do
each of these problems two different ways. Okay.
So, let's get started. Here is the first equation I'm going to be working with.
Two moles of H2O (water) makes
two moles of hydrogen gas and
oxygen here. If there is no coefficient in front of the oxygen,
in front of one of these chemicals we know that it's really
one, that it means one mole, so hey if it helps you
go ahead and write that one in. There's our equation.
Here is the question: how many moles of
O2 (of oxygen) will be produces
from 6.2 moles
of water. Okay, so first of all I want you to think of this like a recipe
and right now it's saying that we start with two moles of this
and that gives us two moles of this and one mole of this.
But, we're not talking about starting with
two moles of H2O, we're talking about starting with 6.2 moles of H2O.
Okay, so this is just like when you're cooking: and you've got to double, triple, or quadruple a recipe.
What do we have to do to this recipe
so that instead of starting with two moles of water we start with 6.2 moles of water?
Okay? We have to multiply everything in this equation
by something that's going to get us to 6.2 moles of water.
We can figure this out a couple of ways. The first thing that I can do
is I can do 6.2, which is this,
divided by 2, which is that, and that
gives me 3.1, okay. Now that gives me the
factor and what I mean by that is that we take
each one of these numbers and we multiply it
times 3.1. Two times 3.1
and now we get 6.2
moles of this. Okay, so now we're starting with 6.2 instead of 2 we
multiply this by 3.1. Now we also want to multiply
by the number of H2 times 3.1, so now we're getting
6.2 moles of H2. And we want to take this number, which is one,
times 3.1, and multiply it
there, and we get 3.1 moles of O2.
So once again we treat this like a recipe in the kitchen:
we're doubling or tripling. Instead of multiplying by 2, or 3, or by 4,
we multiply everything in this recipe by 3.1.
And we get 6.2 here so that's where we're starting with; and we get 6.2 moles
of H2 and we get 3.1
moles of O2. And that kinda makes sense right because
there's this 2 to 1 ration of water to oxygen. We start with 2 of these
we get 1 of these. So this is half of what we have here.
Okay, so if we start with 6.2
we multiply everything by 3.1. We get 6.2 here
and this 3.1, is half what we had over here.
Okay, so that's how we could do this equation
treating it like I a recipe in the
kitchen. Now, let's look at how we can do this using
conversion factors. Okay, what we're going to do is we're going to start with
6.2 moles
of H2O. Now what I want to do is I want to write a conversion factor
that tells me the relationship between the moles of water
and the moles of oxygen. Okay, we can sum it up
with this. It tells us that for every 2 moles
of H2O I get 1 mole of
O2 Two here, one here. Okay?
We get this relationship and what I can do is
I can do is I can use this to write a conversion factor. There are two different conversion factors I can do.
I can write 2 moles of H20
over 1 mole of O2, okay, that's one. Or
I can flip it I can do 1 mole of O2 over two moles of H2O. Of course I"m just getting
the numbers from the chemical equation: 2 in 1. So I've got these
two conversion factors; that are flips.
I can use either one - they're both equally good.
The one that I want to choose though is the one that
I can multiply by this, and cancel out
moles of H20.
That's going to be the one that has H2O
on the bottom, because it's on the top over here. So I'm not going to use this one.
I am going to use this one. I"m going to multiply here
and now I have H20 on the top, H2O on the bottom
so they cancel out. And when I do the math:
I'm going to do 6.2 times 1
divided by 2 equals 3.1
moles of O2. Check it out,
I use a conversion factor method and I get the exact same answer
that I got when I use this like a recipe.
Okay? So the recipe or the conversion factor
both give the same number.
Okay, let's go on to the next one.
Okay, so how many moles of H2O will be required to make
19.2 moles of O2?
Let's treat this like a recipe first okay?
Right now this is a recipe for 1 mole of O2.
What are we going to have to do to this recipe? Double it, triple it,
quadruple it is what I mean.
What are we going to have to do to this recipe to make it a recipe
for 19.2 moles of O2?
Well there's a one right here; one mole of O2 in the recipe.
So it shouldn't come as a shock we have to multiply the recipe
by 19.2.
And now we multiply by 19.2, it will become a recipe
for 19.2
O2, times
19.2 and this is going to to give me
38.4 moles
of H2. Multiply this
19.2 because just like when you're cooking right you've got to
multiply everything in the recipe by the same number.
And this is going to give us 38.4 moles
of H20. This is what happens to the recipe when we
size it up to cook for 19.2 moles of O2.
So the answer is who many moles of H2O are we going to need? We're going to need
38.4 moles of H2O.
That's the recipe method. Okay.
Now let's look at the conversion factor method.
Just as before we realize the relationship between water
and oxygen by using careful equations.
Here is two of these to one of these.
So we can get these two conversion factors.
1 mole of O2 to 2 moles of H2O or 2 moles of H2O
to 1 mole of O2. I'm going to start here with
19.2 moles
of O2, and I want to be able to multiply this by
the conversion factor that's going to cancel out
my moles of O2. So I can choose
this one here because it has moles of O2 on the bottom
cancel out, cancel out. I do 19.2
times 2, divided by one equals
38.4 moles
of H2O. I know it's moles of H20 because that's the unit that I have remaining.
Okay, so sometimes people ask why are you doing it both ways?
Why are you doing it the recipe way
and then the conversion factor way? Okay, the reason is because
for me, the recipe way makes a lot more sense.
But teachers and text books are in love
with conversion factors - they LOVE them! They can't get enough!
I don't think conversion factors make any sense all
alright, but just because they're commonly used
a teacher's going to ask you to do it, a text book is going to ask you to do it
that's why I'm doing it too. But, what I want you to focus on is
actually WHY we're doing it the way we
multiply things and get the answers here with
pretending it's a recipe. I want you to focus on this so you can
understand what's going on. As you can always see, the answer that we get using the
conversion factor method
is the same as the answer that we get when we're treating it
like a recipe and multiplying everything by some number. Okay?
Alright, let's move on to the next question.
So, 2 mole sof H2S (hydrogen sulfide) combined with
3 moles of O2 (oxygen) to make 2 moles
of So2 and 2 moles of H2O.
Using this equation, we're asked, how many moles of O2
are needed to combine with 8.4 moles
of H2S? Okay. What do we have to do with our recipe
to cook with 8.4 moles of H2S
instead of 2 moles of H2S?
What we're going to want to do is we're going to
multiply this by some number that's going to give us
8.4. How can we figure out what that is? Well you might sort of be able to do it
in your head or you can do 8,4
divided by 2 and that's going to give us
4.2. So 4.2
is the number we're going to have to multiply everything by
so we can move from 2 moles of this
to a recipe that uses 8.4
moles of this Okay? So everything
we're going to multiply by 4.2.
Okay, so where we have 3 times 4.2 and that's going to give us
12.6 moles
of O2.
2 to 3, 8.4 to to 12.6
Alright, let's see how we can do this using conversion factors.
We'll start with 8.4
moles of H2S
and multiply that by one of these two conversion factors.
These conversion factors are just telling us
the relationship we have. Here are 2 moles of H2S
to 3 moles of O2.
I can use this relationship to make these two conversion factors.
Which one do I want to use?
I want to use the one that has H2S on bottom so that it cancels out.
So I choose this one. Cancels out.
Cancels out. And this is going to give me -
what a shocker - 12.6
moles of O2. Why moles of O2? Because that's the unit that's left here.
That's how we can do it treating it like a recipe. Here's how we can do using the conversion factor.
Here's the next mole ratio problem. Look, maybe you're getting the hang of these,
maybe you feel great. Turn the video off then and move onto the next thing.
But, if you'd still like some more practice, hey, let's party all night long with mole ratio problems.
Keep doing them until it feels really comfortable and you can
and you're sure you could do any question that's thrown at you.
Okay, starting with 9.2 moles of O2,
how many moles of H2S will you need
and how many moles of SO2 will you get?
Okay, so starting at 9.2 moles of O2
right now our equation is written to start with 3 moles of O2.
So what are we going to have to do? We have to find out how to double, triple, quadruple -
whatever we want to do to this - to get it up from 3 moles of O2
to 9.2 moles of O2.
What am I gonna have to multiply it by to turn this 3 into a 9.2? I can figure that
by doing 9.2 divided by 3
and I'm going to get 3.1.
So, 3 times 3.1
is going to give me 9.2. I'm kind of rounding here.
If you do the math it's not exactly right okay? Anyway
times 3.1 is giving me 9.2 moles of O2.
And how many moles of H2S am I going to need?
Well, I'm going to have to multiply everything in the equations by the same amounts
so times 3.1 here is going to give me
6.2 moles of H2S, so that's how much H2S I'm going to need
to combine with my 9.2 moles of O2.
And then, how many moles of S02 will I get?
I take my 2 moles of SO2 and I multiply that
by 3.1 as well. I've got to do the same thing for the whole equation.
And then I get 6.2 moles of SO2.
So that's how we do that. Let's look at how we do the conversion factors.
You guys are probably really getting the hang of this now, so I'm not going to show all the conversion factor steps.
We start with 9.2 moles of O2
times - what is our relationship between O2 and H2S?
We have 3 moles of O2
to 2 moles of H2S, so I want to write that as a conversion factor
with O2 on the bottom so that cancels out.
So, 3 moles of O2
to 2 moles of H2S
moles of oxygen up here cancel out, moles of oxygen there cancel out,
and I get the same answer that I got here.
I get 9.2 moles of H2S
and now let's just show how we can do this with the moles of SO2.
It's going to be the exact same thing.
I'm not even, like, going to get rid of this thing that's all cancelled out here
I'm going to multiply it by the equation that shows
a conversion factor that relates oxygen here
and SO2 here. Okay? So again
3 moles of this to 2 moles of this, so
3 moles of O2 goes on the bottom,
2 moles of S2O goes on the top -
Cancel this out already, this is cancelled out, and I'm going to get (after I do the math)
9.2 times 2 - divided by 3
equals 6.2 moles of SO2.
Sometimes I write moles with an 'e' sometimes 'mol' because yeah, sometimes I get lazy.
Okay, so that's that. Let's do 2 more - if this feels good
turn it off and move on.
So here we have an equation that talks about
propane burning - which is C3H8 -
nothing in front of this so we can put a one here. 1 mole of this
plus 5 moles of O2 gives me 3 moles of CO2 and
4 moles of H2O. My question here asked me how many moles
of oxygen are needed to react with 7.2 moles of propane.
What do we have to do to this equation to size it up
so that we're starting with 7.2 moles of propane?
Right now we're starting with 1 mole of propane, okay, so this isn't the hardest thing in the world.
We're going to have to muliply this whole equation
times 7.2 and that's going to give us
7.2 moles of propane. How many moles of oxygen are needed?
Well, we have to mulitply it by 7.2 as well.
And that math is going to give me 36.0
moles of O2.
Let's look at conversion factor method. We're going to start with 7.2 moles of C3H8
and we're going to multiply that by conversion factor
that says 1 mole of C3H8 to 5 moles of O2.
Which is going to be on the top? Which is going to be on the bottom?
This is on the top so we want this in the conversion factor to be on the bottom.
Say 1 mole (I'm getting it from right here) 1 mole C3H8
below 5 moles of O2
this cancels out it's on top, this cancels out it's on the bottom
and it gives us 7.2 times 5 divided by 1
equals 36.0 moles of oxygen.
Okay, one more, our last one!
How many moles of propane are needed to make 13.5 moles of CO2?
In order to do that, how much O2 will be needed?
Okay? What do we have to do to our equation right now
in order to make it 13.5 moles of CO2, instead of 3 moles of CO2?
We have to multiply it by some number
it's going to move it from 3 up to 13.5.
And what I can do is I can do 13.5 divided by 3
and that's going to give me 4.5.
So I can mulitply this here by 4.5
and that'll turn my 3 into a 13.5 moles of CO2.
So then when I'm talking about how many moles
of propane I'm going to need I multiply it by the same amount
and I get 1 times 4.5 moles C3H5.
And then I'm asked how much O2 am I going to need?
Well, I have to do the same thing to
all the pieces my recipe; so 5 times 4.5 is going to give me
22.5 moles of O2.
This is what happens to my recipe
when I increase the size of it. Let me just show you these really quickly.
I'll start with 13.5 moles of CO2
times a conversion factor that talks about the relationship between
propane (C3H8) and CO2 here.
CO2 is going to be on the bottom, so that it cancels out.
So, 3 moles of CO2 on the bottom
1 mole of C3H8 on the top
13.5 times 1 divided by 3
is going to give me 4.5 moles of C3H8.
The number is the same even though the process is different
Oh what did I forget to do? I forgot to cancel these out
or I could - I'm not going to get rid of it
I'm going to keep it there or to find out how much O2 I'm going to need
Well what is a relationship between CO2 and 02?
It's 3 to 5. So 3 moles CO2 on the bottom
and 5 moles of O2 on the top.
These cancel out
13.5 times 5 divided by 3 equals 22.5 moles O2.
Same answer regardless of whether I do it the recipe method
or the conversion factor method.
If you made it all the way to the end of this video, you partied with me all night long
on mole ratios you're a rockstar. But, more than anything, I bet this
really makes sense and I bet you can tackle any mole ratio problem that's thrown at you.
Feel free to use recipe method or the conversion factor method unless
your teacher says you have to use a conversion factor method.
Then go ahead and use it but know in the back of your mind
what is actually going on and why you're multiplying by what you're doing
and all that sort of stuff. All right? Good luck!