I have been reading the answer of this question here- Why is weight of 1 mole of substance equal to atomic/molecular mass in grams

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I feel that the answer writer was wrong to say that -"the ratio of g/mol to amu/atom is exactly one"

I feel so because he proved that the ratio of g/mol C12 to amu/atom C12 is exactly one.He never proved that the ratio of g/mol to amu/atom is exactly one.If I am wrong then please correct me!


1 Answer 1


The statement is more or less correct.

But there are two caveats here:

  1. There is no proving of these concepts since there are definitions.

  2. In general, it's a bit strange to use the ratio of amu to atom. An atom has a well defined mass based on it's isotope, but the number of grams per mole is defined based on natural abundance and weighted average mass. Therefore, there is some discrepancy here. Of course, here, you're referring specifically to carbon-12, so there is no confusion, but just watch out in the general case.

  • $\begingroup$ Why is it that 12 grams of C12 atoms represent 6.022* 10^23 atoms of carbon? Similarly why 1 g of H represents 6.022*10^23 atoms of H? $\endgroup$
    – user419155
    Mar 9, 2017 at 17:59
  • 1
    $\begingroup$ That comes from the definition of mole and Avogadro's number. en.wikipedia.org/wiki/Avogadro_constant $\endgroup$
    – Zhe
    Mar 9, 2017 at 18:37
  • $\begingroup$ According to Wikipedia- Mole is defined as the amount of a chemical substance that contains as many representative particles, e.g., atoms,molecules, ions, electrons, or photons, as there are atoms in 12 grams ofcarbon-12 (12C), the isotope of relative atomic mass 12 by definition. But this definition has nothing to do with 6.022*10^23.How should I establish the relation between the number of atoms in one mole of C12 and it's gram atomic mass (that is 12 g,numerically equal to its atomic mass)? $\endgroup$
    – user419155
    Mar 10, 2017 at 2:00
  • $\begingroup$ Guess how many atoms of carbon-12 are in 12 grams of carbon-12... $\endgroup$
    – Zhe
    Mar 10, 2017 at 3:48
  • $\begingroup$ It is 6.022*10^23 atoms.I found it written on wikipedia.however,I do not know ,how chemist got to the point that 12 g of C12 atom always contain 6.022*10^23 atoms??have they assumed it? $\endgroup$
    – user419155
    Mar 10, 2017 at 4:16

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