Yes I am looking for some homework help, Chemistry was never my course.  Finally made it into my Mechanical Engineering Thermodynamics class and it deals with too much chemistry.

The whole idea of converting from lb mol to lb mass just boggles me, Im waiting to see if my professor will schedule me an office visit before this homework is due but he's bad about communication so Im hoping someone here may be able to help me.

Determine the mass in lbm of 1.00 lbmole of Illinois Coal having the molecular structure C_100 H_85 S_2.1 N_1.5 O_9.5    When I type a letter subscript a number I'm trying to show the formula so water would be H_2 O.  Just to be clear.

A pound mole is the atomic mass in pounds.  So a pound mole of water is 18 pounds.  Your compound is C_100 H_85 S_2.1 N_1.5 O_9.5.  That means that the molecular weight is 100*12.01+85*1.008+2.1*32.07+1.5*14.01+9.5*16.00 or 1527.  That means that a pound mole of your compound would be 1527lbs.
 

Thanks guys,  I made it in and talk to me professor and he confused the living hell out of me.  I guess it makes sense and I do understand it now that I think about it but yes back when I took chemistry it was always mols not lb-mols..  

So I don't need to really convert anything?  Just find the weight, multiply it by how ever many lb moles I am given and that is how many pounds of what ever substance I have?

This class is just blowing my mind and we are only a week in.  its taking everything I know and for the most part throwing it out the window and making me learn new similar ways.

Yes.  Lets do it for a simple on like water.  Water is H2O.  Hydrogen has an atomic weight of 1 and Oxygen has an atomic weight of 16.  That means that water has a molecular mass of 18.  If you have 1 lb mole of water that means that you have 16 lbs of water.

Yah, I am very familiar with the gram mole and kilogram mole.  It's just that I had never seen any one commit the heresy of using an imperial unit.

 

What I find interesting is that many mechanical engineers insist on using the arbitrary unit of a pound-mass.  A pound in my world is a weight, while the slug is the unit of mass, like the gram or Kilogram.  If one wants to use the USCS of units, then the slug-mole makes more sense.   If you use the slug, then everything is like the SI units.  Can you imagine using the newton-mole instead?  That is the equivalent of using a pound-mole.

I still don't freaking understand when to use lb-mass or slugs... Some classes they make us use slugs others like this one its lb-mass.. I wish we could just use SI, for calculations sake I find it so much easier!  

When you deal everyday with things like enthalpy (units in Btu/lbm), then lbm becomes convenient for calculations.  At least in my industry (US nuclear power), the standard English units are unlikely to go away anytime soon.  Don't get me wrong, I enjoy the simplicity of SI, but it's simply not convenient for me to give information to customers in that manner.

For me at least, slugs really only come into play when messing with viscosity.  Almost fitting, right?

To sidetrack this thread a bit further, when you think in terms of pounds as being a force only, and the slug as the unit of mass, then everything in USCS is pretty much the same as SI.  Of course it is not quite as easy as the base 10 stuff of SI, but one approaches the formulas and calculations the same.

Can you imagine the classic problem of a truck going down a hill and how long it will take it to stop if you are given the weight of the truck in newtons instead of its mass in Kilograms?  That same problem is given in USCS units where the weight of truck is given in pounds in an effort to show the deficiencies of the USCS.   There is really no difference in F=mA and the like equations when one uses the correct units.

However, there is the made-up unit of Kg force that is used in things like Brinell hardness scale.   This makes as much sense as a slug weight or the pound mass, or . . . .  Just keeps the units what they are suppose to be and things are pretty much the same.

Ugh, I remember reading through some old research paper recently (I want to say it was old Russian info on orifice pressure drop or something) and they used kg-force.  WTF.  Why fuck with the simplicity of SI?

How many scales have you seen that measure Newtons?

None that I can think of, but I live in 'murica.

They are no tin Europe either.
Scales measure kilograms.

The pound-mole is because the units for the weight on a periodic table used to compute the mass of a mol are arbitrary.
We used to have Carbon and Oxygen based tables.
Carbon was 12.000 or oxygen was 16.000 amu. Other elements then tracked from there.

My HS Physics teacher had one.  He was a huge nerd and an SI Nazi.  

 

Ten years as a chemist in the field of developmental organic synthesis and I've never heard anyone express a desire to know a quantity expressed in terms of "pound moles". I'm also relatively sure any of my colleagues would merely stare unbelieving and possibly blink their eyes once or twice in an attempt to elicit a "Just Trolling you Brah!" response from the inquisitor. Way more concerned with things like gram mass and yield.

Sometimes academia strays from real world utility to the point of belying it's obliviousness to same.

Also, this:



The answer Doctor Professor is 65.

Q.E.D.

Haha thanks for the entertainment all!

This class has ended up being a real pain in my ass.  One day it is easy, almost suspiciously easy.  Next day it says 2+2=4  Now calculate the mass of the sun using the color red.



Ive just learned to accept that instead of slugs I am now supposed to use lbf and for mass lbm...  Ok.

Teacher through us a curve ball on an exam today... he gave us all of our pressures in psig instead of psia.. He almost got me then I caught onto what he did.  

It's like "slugs" in Physics....



Seriously....who in the bloody hell uses slugs?