For this reason, the term "mole" was adopted to signify the atomic, molecular, or formula weight of a pure substance expressed in grams.Alternative definition of the mole: O, weighs 46.069 g. If we mix 46.069 g of ethyl alcohol with 18.015 g of water, we can be assured that the mixture contains 1 molecule of ethyl alcohol per molecule of water.Because the atomic weight scale is numerically preserved in the definition of gram atomic weights, the mass of 1 gram-atomic weight of any element could be immediately determined as the atomic weight in grams.
This is not worrisome, because neither number is crucial to the utility of the mole. It is used to represent the mass of a single atom, molecule, or formula unit of a substance, in which case it has units amu/atom.
It is also used to represent the mass of a mole of substance, in which case it has units g/mole.
It is nearly analogous to defining a dozen as the mass of a substance that contains the same number of fundamental units as are contained in 733 g of Grade A large eggs.
This definition completely obscures the utility of the dozen: that it is 12 things! The mole is the same kind of unit as the dozen -- a certain number of things. First, the number of things in a mole is so huge that we cannot identify with it in the way that we can identify with 12 things.
In practice, we seldom need to know how many atoms or molecules we are working with, so in mole calculations the number 6.022 x 10 is rarely used.
What is invariably used (except for sample calculations in chemistry textbooks; see below!
It is very nice that we know it; but we do not need to know it for the mole to be useful.
I would even go so far as to say that the modern definition of the mole in terms of a certain number of atoms of things/mole, must be used in any and every calculation involving moles!
The atomic weight scale defines the masses of atoms relative to the mass of an atom of C, which is assigned a mass of exactly 12.000 atomic mass units (amu).
The number 12 is chosen so that the least massive atom, hydrogen, has a mass of about 1 (actually 1.008) on the scale.