[user11]

Hi, Can someone explain to me in detail (prefereble with examples) what relative atomic mass means and WHY it's used?  Why did some scientist say that the relative mass of C-12 is 12? Why is it so useful? And how did the person do it? Why don't we use hydrogen or Oxygen as a relative atomic mass instead of C-12? 

[Arkcon]

We have used oxygen and hydrogen as the definition of atomic mass before.  It is, pretty much, an arbitrary decision.  If you try a search on this forum, with relative atomic mass in the title, you will find this question has been asked by noobs again and again.  I'm afraid it will disappoint you to learn, that there is no good answer to your very general question.  As an example:

http://www.chemicalforums.com/index.php?topic=67951.msg244767#msg244767