Welcome to Mole Conversions Made Easy, brought
to you by Ketzbook.
In this video, we are going to learn how to
do mole conversions for elements, how to calculate
the molar mass of a compound, and how to do
mole conversions for compounds.
But before we jump into all that, you might
be wondering...what is a mole?
Well, there are lots of different moles in
the world, but in chemistry, a mole is simply
a large number of things.
It’s kind of like a dozen, only bigger.
One dozen is 12 things.
One mole is 6 times 10 to the 23rd things,
that is, 600 billion trillion things.
Now, that’s a lot of things!
But why is the mole such a big number?
Because atoms are so small.
Suppose you wanted to know how many hydrogen
atoms are in one cup of water.
If you were able to count all the atoms, you
would find that there are about 15 trillion
trillion hydrogen atoms in a cup of water.
But if we count using moles instead, that
works out to be only 25 moles of hydrogen
atoms.
So, we use moles to count atoms, molecules,
and other chemicals.
It turns out that there is an even better
reason why 1 mole equals 6 times 10 to the 23rd things.
And that is that one gram equals 6 times 10
to the 23rd atomic mass units.
Let’s see what this means for a particular
element.
Break out your periodic table, and look for
lithium.
It is the third element.
On the bottom of the square, you should see
the number 6.94.
This is the atomic weight of lithium.
But it is also the molar mass of lithium.
So, one lithium atom has an average mass of
6.94 amu, AND one mole of lithium atoms has
a mass of 6.94 grams.
That give us a convenient way to count atoms
by weighing them, and the molar mass is the
conversion factor between grams and moles.
Let’s try a problem.
How many moles of lithium are in 25 g of lithium?
This is a one step unit conversion problem,
and like any unit conversion problem,
the first thing you should do is write down the
quantity that you know...
in this case, 25 grams of lithium.
Next, multiply this by a conversion factor
fraction.
Remember that for mole conversions, the molar
mass is always our conversion factor.
One mole of lithium equals 6.94 grams.
Because we started with grams, we put the
6.94 grams on the bottom of the fraction.
Grams on the top and bottom cancel each other
out.
Next, write one mole on the top of the fraction.
Because the one is on the top of the fraction,
this becomes a division problem.
In your calculator type 25 divided by 6.94.
The answer is 3.6 moles of lithium.
You may have been wondering, what happened
to the “e” in mole?
Well, it turns out that the abbreviation of
mole is M-O-L.
Isn’t it just wonderful how much energy
we are all going to save by not writing the “e”?
Okay, time for another sample problem.
What is the mass of 11.5 moles of lithium?
Before we solve this problem, we realize that
mass is measured in grams,
so we need to convert from moles to grams.
The first thing you should do is write down
the quantity you know, 11.5 moles of lithium.
Next, multiply this by a conversion factor
fraction.
The molar mass of lithium is still the conversion
factor.
Because we are starting with moles, one mole
goes on the bottom and 6.94 grams goes on the top.
Moles on the top and bottom cancel each other
out.
Because the one is on the bottom of the fraction,
in your calculator type 11.5 times 6.94.
The answer works out to be 79.8 grams of lithium.
That is how to convert between moles and grams
for elements, but what about molecules and compounds?
Let’s start by looking at molecules made
from carbon, nitrogen, and oxygen.
The molar mass of each element is typically
written on the bottom.
Now in order to convert between grams and
moles for a molecule, we will need to calculate
the molar mass of the molecule by adding up
the molar masses of all the atoms in the molecule.
Let’s try a few examples.
Carbon monoxide has one carbon and one oxygen,
so we add 12.01 for carbon and 16 for oxygen
to get a molar mass of 28.01 grams per mole.
In general, the units of molar mass are grams
per mole.
However, it is very useful to write the molar
mass as an equality.
One mole of carbon monoxide has a mass of
28.01 grams.
That helps us to remember that the molar mass
is a conversion factor.
Let’s try another molecule.
Nitrogen is a diatomic element composed of
N2 molecules.
Because there are 2 nitrogen atoms per molecule,
we multiply 14.01 by 2,
so the molar mass of N2 is 28.02 grams per mole.
For carbon dioxide, there is one carbon and
two oxygen atoms.
So, we add 12.01 for carbon and 16 times 2
for the two oxygens,
which gives us a molar mass of 44.01 grams per mole.
We can do the same thing for ionic compounds,
like magnesium nitrate.
The molar mass of magnesium is 24.3.
Because there are two nitrates in the formula,
that means we have 2 nitrogen atoms and 6 oxygen atoms.
So, we add 24.3 plus 14.01 times 2 plus 16
times 6,
which works out to be a molar mass of 148.3 grams per mole.
Remember that we can write the molar mass
as an equality, so for magnesium nitrate,
1 mole equals 148.3 grams.
That is our conversion factor between grams
and moles.
So let’s go ahead and use this molar mass
to do some mole conversion problems.
Let’s convert 6.35 grams of magnesium nitrate
to moles.
We will solve this problem in exactly the
same way that we converted grams of lithium to moles of lithium.
First, write down the quantity that we know,
6.35 grams of magnesium nitrate.
Next, multiply this by a conversion factor
fraction.
The molar mass of magnesium nitrate is our
conversion factor.
Because we started with grams, write 148.3
grams on the bottom.
Because we are solving for moles, write one
mole on the top.
Grams on the top and bottom cancel out.
Because the one is on the top, in your calculator
type 6.35 divided by 148.3.
This works out to be 0.0428 moles of magnesium
nitrate.
What if we want to know the mass of 0.369
moles of magnesium nitrate?
First, write down the quantity that we know,
0.369 moles of magnesium nitrate.
Next, multiply this by a conversion factor
fraction, using the molar mass as the conversion factor.
Because we are starting with moles, put 1
mole on the bottom;
and because we are solving for mass, put 148.3 grams on the top.
Moles on the top and bottom cancel each other
out.
Because the one is on the bottom of the fraction,
multiply 0.369 times 148.3,
which gives us 54.7 grams of magnesium nitrate.
Thanks for watching.
If this video helped you at all, please give
me a thumbs up.
It means a lot to me.
Feel free to also share any comments or questions
you have below, subscribe to my channel,
or check me out at ketzbook.com.
