Second, 12 is an important number in the English system of weights and measures, so the definition of a dozen as 12 things makes sense.However, the choice of the unusual number, 6.022 x 10 is used, it is necessary to resurrect an older, in some ways more sensible and useful, definition of the mole, which is grounded in the atomic weight scale addressed above.
This is a very useful correpondence, but it was discovered only after the weight scale had been in use for a long time.
In their desire to be able to count atoms by weighing, chemists gradually developed the concept of the "gram-atomic weight", which was defined in exact correspondence with the atomic weight scale: 1 atom of C weighs 12.000 g Thus the gram-atomic weight of an element was defined as the atomic weight of the element, expressed in grams.
Many chemists prefer to use the term molar mass for the mass of a mole of substance.
In this course, we will use the phrase Formula Weight for both situations. The importance of the mole concept can be summed up as follows: any statement that can be made about the number of atoms of an element in a molecule or formula unit of a substance can also be made about the number of moles of an element in a mole of the substance. Convert moles of S to mass of S using the atomic weight.
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.Although we are confident that we know its value quite well, some future experiment may cause us to make a small revision in the number.By necessity, the mass of 1 m in grams will change accordingly.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!The early versions of the atomic weight scale were established by scientists who had no knowledge of the electron, proton, or neutron.When these were discovered in the late 19th and early 20th centuries, it turned out that the mass of an atom on the atomic weight scale was very nearly the same as the number of protons in its nucleus.) is the fact that 1 mole of substance is its formula weight in grams.The dual definitions of the mole can be used to find the mass of 1 amu expressed in g. Avogadro's number is an experimentally measured quantity. It provides a bridge between the atom and the macroscopic amounts of material that we work with in the laboratory.It allows the chemist to weigh out amounts of two substances, say iron and sulfur, such that equal numbers of atoms of iron and sulfur are obtained. Unfortunately, the clumsy definition of the mole obscures its utility.