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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!

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user419155
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1 Answers1

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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.

Zhe
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  • 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? – user419155 Mar 09 '17 at 17:59
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    That comes from the definition of mole and Avogadro's number. https://en.wikipedia.org/wiki/Avogadro_constant – Zhe Mar 09 '17 at 18:37
  • 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)? – user419155 Mar 10 '17 at 02:00
  • Guess how many atoms of carbon-12 are in 12 grams of carbon-12... – Zhe Mar 10 '17 at 03:48
  • It is 6.02210^23 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 6.02210^23 atoms??have they assumed it? – user419155 Mar 10 '17 at 04:16
  • Again, that is the definition of mole. We know that we have a constant number of units due to the law of definite proportions. Measurement is another issue. See: https://en.wikipedia.org/wiki/Avogadro_constant#Measurement – Zhe Mar 10 '17 at 14:09
  • ok!but why did chemists decide to use the number 6.022 * 10^23 as a mole?was there any specific reason? – user419155 Mar 10 '17 at 14:45
  • We didn't decide. We measured it... – Zhe Mar 10 '17 at 14:45
  • I think i am in a big confusion! – user419155 Mar 10 '17 at 14:46
  • Let me start from beginning. – user419155 Mar 10 '17 at 14:46
  • Wikipedia gives this definition of a mole - A 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 carbonwith relative atomic mass 12 by definition. so,my first question is that how did chemists come to know that 12 grams of C12 atom would contain 6.022 * 10^23 atoms? – user419155 Mar 10 '17 at 14:50
  • You can perform experiments, see comment above: en.wikipedia.org/wiki/Avogadro_constant#Measurement – Zhe Mar 18 '17 at 15:56