mass
'no magic to the mole'
amount
molar mass
concentration
solution volume
gas volume
molar gas volume
Avogadro
constant, L
number of
entities, N
Now try the following question.
2.2 RELATIVE ATOMIC MASS, A
r
Traditionally, the masses of atoms have been expressed in terms of relative mass. While Wenzel (1777), Richter (1792), and Dalton (1808) were those first able to establish any semi-meaningful data, they were working with a limited number of (the known) elements, no knowledge of atomic structures or the particles responsible for atomic mass or those involved in chemical bonding, and they could not possibly have imagined the existence of isotopes for the majority of elements.
An atom of H was the chosen standard for the original atomic mass scale because H, being the lightest atom, was also considered to be the simplest. It was convenient to have all relative atomic masses ≥ 1.
For a number of reasons H was abandoned as a standard, not least because so few elements combine with it directly and, early in the c.20th, a new standard, based on oxygen, was introduced.
However, far more accurate masses of atoms became available routinely with the development of the instrument called a mass spectrometer (F.W. Aston, 1919). Immediately following this it was demonstrated conclusively the existence of isotopes for neon gas and subsequently for many other chemical elements.
In 1929 it was discovered that not only did
oxygen gas - like most elements - comprise isotopes, but also that the composition of the mixture of isotopes varies very slightly depending upon its source or origin. So therefore must the average mass of the oxygen atom that was used as
