Glen Thorncroft
Q. Why is the molecular weight the same value whether it’s g/mol, kg/kmol or
even lbm/lbmol?
A. You should, of course, review your Chemistry textbook, but even it may not explain this one.
Okay, let’s start with basic Chemistry. What is a mole? It is 6.02 x 1023 of something: atoms,
molecules, or any other elementary units you may want to define. Why such an odd-looking number?
Because it is the number of atoms in 12 grams of Carbon-12. It’s a measured quantity, but basing the
value of the mole on the mass of Carbon, as opposed to any other element, is arbitrary. Recall that this
number is called Avogardo’s number, and is used to determine atomic masses and proportions in
chemical formulas.
So how much mass does 6.02x1023 atoms of Carbon-12 have? By definition, 12 grams. Or we
can say that the atomic mass of Carbon-12 is 12g/mol. [note that the symbol for mole is mol.]
Okay, so what’s a kmol? It’s called a “kilogram-mole,” and is the number of atoms in 12
kilograms of Carbon-12. So by definition, there would be 12 kg in one kmol of Carbon-12.
You can probably predict what the lbmol is. It’s called a “pound-mole,” and is the number of
atoms in 12 pounds (mass) of Carbon-12. So again, by definition, there are 12 lbm in one lbmol of
Carbon-12.
Thus we can see that the values of the atomic or molecular masses are the same, whether the units
are g/mol, kg/kmol or lbm/lbmol.
[Are the number of atoms the same? Think about it…]
Q. Why are these values called atomic weight or molecular weight?
Shouldn’t they be called masses?
A. Yes. Unfortunately we tend to use the terms mass and weight interchangeably when referring to this
chemical property. I did that myself in the above discussion. Just remember that, whatever you call it,
we’re really talking about mass.