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 6.023×10^23 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.