1 amu (atomic mass unit) is defined as the one twelfth of the mass of a carbon-12 atom. The mole is defined as the number of carbon-12 atom in 12 g of carbon-12. In other words,

$$\require{cancel} 1 \textbf{ mole} = \dfrac{12 \textbf{ g}}{m(^{12}C)} = \dfrac{\cancel{12} \textbf{ g}}{\cancel{12} \textbf{ amu}} = \dfrac{\textbf{ g}}{\textbf{ amu}} $$

I believe that the 12 g were chosen so that the mole could exacly express the ratio between the g and amu.

Consider now this alternative definition : the amu is defined as the exact mass of a proton and the mole is exact ratio between the g and the amu. Wouldn't it be simpler this way?

What are the advantages of one definition over the other? Is there any reason we would want to keep the former? Why scientists decided to define the mole like that and not pick a simpler definition like the one I proposed?

  • $\begingroup$ I wasn't very clear. See my edited question :) $\endgroup$ – Corvinus Apr 11 '17 at 16:09
  • $\begingroup$ Ah, ok. That's slightly different. $\endgroup$ – orthocresol Apr 11 '17 at 16:14

One of the reasons to take Carbon-12 could be that it's hard to get your hands on one mole of pure hydrogen (and to weigh it). Carbon-12 is a (room temperature) solid which is relatively easy to produce, keep pure, store and weigh.

  • $\begingroup$ I believe that the mass of a proton has been derived experimentally with high precision. Why not define this as 1 amu = 1 g/mol and then express the mass of every atom accordingly? I don't understand where is the need to either weigh Carbon-12 of pure hydrogen. $\endgroup$ – Corvinus Apr 11 '17 at 10:39
  • $\begingroup$ @Corvinus - In addition to altering the molar masses of everything, the definition of the Avogadro constant is dependent on the definition of the unified atomic mass unit. Changing the definition of $\mathrm{u}$ would change the definition of $N_\mathrm{A}$, which would change the definition of the ideal gas constant $R$, since $R=N_\mathrm{A}k_\mathrm{B}$ (the latter term being the fundamental Boltzmann constant). $\endgroup$ – Ben Norris Apr 11 '17 at 15:19
  • $\begingroup$ Actually, since $R$ is measurable (and $k_\mathrm{B}$ is not so easily measurable), changing $\mathrm{u}$ would change the Boltzmann constant, and all chemists and physicists would be upset. $\endgroup$ – Ben Norris Apr 11 '17 at 15:21
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    $\begingroup$ I understand that it wouldn't make any sense to change the definition now. I see now that I wasn't very clear in my question : what I would like to understand is why scientists decided to define the mole like that and not pick a simpler definition like the one I proposed? (I'm editing my question to make that clearer) $\endgroup$ – Corvinus Apr 11 '17 at 16:03
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    $\begingroup$ I seriously doubt that any appreciable quantity of pure $\ce{^{12}C}$ exists. The mass of $\ce{^{13}C}$ is known to 13(?) significant figures and is about 1% of non-radioactive carbon. So you'd have to have $\ce{^{12}C}$ pure to about 12 significant figures to not perturb the mass. That just seems impossible for any reasonable mass of C. $\endgroup$ – MaxW Apr 11 '17 at 18:47

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