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I'mrather the
asking how I would know that the mass of sample is 1 kg. It just
appears to me that we don't count the number of atoms but
concentration of atoms. Am I wrong about that?
The proposal is to count the atoms. I don't see what
concentration has to do with it.
This scheme is predicated on being able to count atoms
accurately. You need to count a *lot* of atoms in order to
construct a secondary mass-standard that is large enough to
be useful. The proposal on the table is to count atoms by
dividing the volume of a crystal by the volume of the unit
cell. The latter can be found by means of xray
crystallography ... we know that the dispersion of a
diffraction grating depends in a simple way on the spacing of
the grating.
> If so, why not just define 1 kg of
silicon in terms of the volume of the sample? Why not usethe mass of
each atom and the configuration of silicon to obtain thedensity and
then define 1 kg in terms of the volume and density of silicon?
You could ... but you would need to think real hard about
what volume to establish as the standard, lest you choose
something that is inconsistent with the previous values of
the kilogram and/or Avogadro's number. Also of course you
would need to specify the conditions (STP or whatever) at
which the density is obtained.
[By the way, if a balance really measures inertial mass, itwouldn't
matter where the object's center of mass is, right? Keepin mind that
the earth's gravitational field decreases with height.]
Is this a significant problem? How tall are the objects you
are weighing?
I suspect other non-idealities e.g. buoyancy are more worth
worrying about.