In Class, there was a discussion of the difference between a lb mole, a g mole and a kg mole. To begin, I would like to first present the explanation given by D.M. Himmelblau in "Basic Principle and Calculations in Chemical Engineering", Prentice Hall, 1982. [This is not an exact quote, but close]

What is a mole? The best answer is that a mole is a certain number of molecules, atoms, electrons, etc. In the SI system, the mole has about 6.023 E23 particles. This is correctly defined as a gram mole, though SI units officially designate this as a mole. Alternatively, a kg mol is equal to 1000 * g mol, and therefore can be thought of as having 6.023 E26 particles. A lb mole [pound mole] has 6.023 E23 * 424 particles and is therefore larger than a g mole.

Another way to look at the mole unit is as a unit used to convert the number of moles to mass, using the molecular weight.

the g mol = [mass in grams]/molecular weight

the lb mol = [mass in lb]/molecular weight


mass in g = (mol wt.) (g mol)

mass in lb = (mol. wt.) (lb mol)


Now, does that clarify or confuse things? I think that the confusion begins because, you, as chemists, think of a mole as 6.023 E23 particles and also that the molecular weight is the mass of 6.023 E23 particles of a substance (in grams). Engineers are not as grounded in specifics and therefore, they may define a molecular weight as the conversion unit for moles to mass. The molecular weight can not change for a substance, so, for there to be different measures of mass (lb, kg, g, ton, etc..), there must be different 'moles'. These various types of moles are denoted by the unit of mass to which they correlate molecular weight. In order to be consistent they must then define the number of particles that make up these different moles as different, thus the explanation above.

As was noted in class, this differentiation is not critical to understanding or even solving the problems. Sometimes, it can be ignored altogether. However, as you enter the industrial world, you will interact more with engineers and an understanding of the differences in terminology is important.

Let me know if this makes sense or not.