In this video we are going to perform unit conversions for molar amounts, in cases where we wanted to
go from say gram-moles to kilogram-moles, or kilogram-moles to pound-moles.
Then you might be thinking why we have all these types of units for moles,
and it comes about by if we're talking about large scale industrial processing. We don't necessarily
talk about how many gram-moles of a product we are producing. Whereas if we are
in a lab and working on a bench top in a small flask maybe a gram-mole is an appropriate unit for
the amount we are working with. So this can be one of the more confusing conversions that
we do, but hopefully after this video, you will see that it is pretty straight forward.
With any conversion we use a conversion factor, and in this case the one that instantly pops
to mind should be molecular weight, which reports the molar mass of an element or species, and
we have seen this in previous chemistry lessons is typically as gram per mole of whatever,
and what does this mean again? This means that we if we had oxygen then we know that
the molecular weight is 32, but that means that it weighs 32 grams for one mole of oxygen, but
what is one mole of oxygen? That is 6.023 x 10^23 atoms. So if we had that many atoms
of oxygen it would have a mass of 32 grams. So here is where it gets fun, if we have 1 gram-mole,
or 1 gram per gram-mole, this is equivalent to 1 pound per pound-mole, and you are thinking, "what? wait!"
This does not imply that a gmol equals 1 lbmol. That is NOT true.
So we know a gmol, we defined that as a certain amount of atoms.
So a lbmol is not that many atoms. What this means it that if we had a certain amount of
atoms that comprise what is called a lbmol, it is going to a mass of a pound.
Alright let's just get straight into an example so we can see how this works. Let's say we have 500 moles
of oxygen and this is entering some imaginary tank and leaving at the same rate. We want
to know how many kgmol/hr, how many lbmol/hr, and how many gmol/hr are leaving this tank. So
for the sake of conversions. A mole can be written as a mol or as a gmol, all the
same thing. So 500 moles an hour of oxygen is equal to 500 gmol of oxygen per hour.
Now if we want to go to say kmol, we know that 1 kmol is 1000 moles. So 500 moles of oxygen
per hour times our conversion factor, one over 1000 moles, gives us 0.5 kmol of oxygen per
hour. So you have crossed off those two, pretty straight forward. Now you will rarely come
across a kgmol, but if you are working with a large quantity in an industrial experience
you may see a kgmol. So how do we convert to kgmol. Let's do it the long way so we can see.
We have 500 moles of oxygen per hour. Now if we multiply this by our molecular weight
of oxygen, which is our conversion factor. 32 grams of oxygen per mole of oxygen we get
16000 grams of oxygen per hour, so now we have a mass flow rate, if that is what we
wanted, but it’s not. So we need to go from this mass to our kgmol. So we can convert
grams to kilograms, and we get 16kg of oxygen per hour, and we can use our same conversion
factor. We can take 16 kg of oxygen per hour divided by our conversion factor, the molecular
weight, which in this case is 32 kg of oxygen per kgmol, we get 0.5 kgmol per hour. So you
should notice something. These have the same value, but a kilogram mole refers to 6.023x10^23
groups of 1000 atoms. If it were a mole it would be ten to the 23 atoms. Which is up
here. Here we are saying we have 6.023 x10^23 atoms and we are using a prefix to ask in
terms of 1000. So basically, they are the same thing. So the last conversion we are
going to do is to convert to lbmol. So we want to know in 500 moles of oxygen how many
lbmol of oxygen do we have flowing per hour. So let's use the same long way we did before where we take 500
moles of oxygen per hour. Multiply it by our conversion factor of 32 grams of oxygen per
mole of oxygen, and we get 16000 grams of oxygen per hour. Now we take that 16000 grams
and convert it to pounds, using the conversion factor of 454 grams per one lb-mass. This
gives us 35.24 lbs of oxygen per hour. Now we can then take 35.24 divide it by our conversion
factor, our molecular mass, 32 lb/lbmol and we should get 1.1 lbmol per hour. Now the
short cut way to do this is by seeing that we are using our conversion factor twice.
So we could just jump those out. To show that the conversion really is just lb to grams
if we take 500 moles of oxygen per hour multiply it by 1 lb over 454 grams and just to make
sure the units look okay. You can see that we get the exact same thing. So to convert
from moles to lbmol we use our conversion of pounds to grams.