B So what is the new definition of the kilogram?

[gmax137]

Avogadro’s number is now exact.

is that the Avogadro constant is unconnected with the definition of the kilogram

Right, like "a dozen" means twelve of anything. Twelve apples, twelve dwarves, twelve eggs. Avogadro's number is a super-jumbo sized "dozen."

By the way, I worked with an older guy who routinely said "six times ten to the 23; Ava Gardner's number" with a straight face.

 

[GuyBarry]

Right, like "a dozen" means twelve of anything. Twelve apples, twelve dwarves, twelve eggs. Avogadro's number is a super-jumbo sized "dozen."

You've got it exactly. The Avogadro constant is expressed in the units mol^-1. You might as well define "dozen" as an SI unit (symbol "doz") and specify the "dozenal constant" as 1.2 x 10¹ doz^-1.

Not really a "unit" in the sense that it's otherwise understood - just a gigantic multiplier.

 

[lpetrich]

I went over to https://www.bipm.org/en/measurement-units/rev-si/ and I found https://www.bipm.org/utils/en/pdf/si-revised-brochure/Draft-SI-Brochure-2018.pdf (5 February 2018).

The previous definition of the mole fixed the value of the molar mass of carbon 12, M(¹²C), to be exactly 0.012 kg/mol. According to the present definition M(¹²C) is no longer known exactly and must be determined experimentally. The value chosen for N_A is such that at the time of adopting the present definition of the mole, M(¹²C) was equal to 0.012 kg/mol with a relative standard uncertainty of 4.5 × 10⁻¹⁰.

So the atomic mass unit will be redefined as (1 gram) / (Avogadro's number), independent of carbon-12.

 

[GuyBarry]

I went over to https://www.bipm.org/en/measurement-units/rev-si/ and I found https://www.bipm.org/utils/en/pdf/si-revised-brochure/Draft-SI-Brochure-2018.pdf (5 February 2018).

So the atomic mass unit will be redefined as (1 gram) / (Avogadro's number), independent of carbon-12.

I really don't think that's correct. If you look at the top of page 29 it says "The dalton (Da) and the unified atomic mass unit (u) are alternative names (and symbols) for the same unit, equal to 1/12 of the mass of a free carbon 12 atom, at rest and in its ground state". So the amu is not being redefined.

Under the new definitions, the relative atomic mass of carbon-12 will remain exactly 12. However, the molar mass of carbon-12 will no longer be exactly 0.012 kg/mol. A consequence of this is that the molar mass constant, which was previously defined at exactly 0.001 kg/mol, will now be determined experimentally. It can still be taken as 1 g/mol for practical purposes. From Wikipedia:

Because the new SI definitions include a new definition of the mole through giving the Avogadro constant an exact numerical value, the value of the molar mass constant will no longer have an exactly defined value, assuming that the definition of a dalton (one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state) does not change. Therefore, while still retaining with great accuracy a value of 1 g/mol, the molar mass constant will no longer be exactly defined at that value.

https://en.wikipedia.org/wiki/Molar_mass_constant