Orchid XP v8 schrieb:
>>> ...and here I was thinking that 12g of C12 was *defined as* one mole 
>>> of C12 atoms...
>>
>>   Hence the definition of 'mole' depends on the definition of 'kilogram',
>> not the other way around.
> 
> Ah, I see.
> 
> Couldn't they just reverse it? I mean, just say that 1 Kg = the mass of 
> XXX C12 atoms?

That's one attempt to it (except that they appear to intend to use 
silicon instead of carbon).

However, at present (or at least until very recently) measuring the mass 
of the Grand K can still be done with greater precision than counting 
atoms, so they don't know /exactly/ how many atoms make up a mole.

Definitions of SI units change over time depending on how they can be 
measured with highest precision.

For instance, the SI unit of distance (1 metre) was, like the kilogram, 
initially (*) defined based on a physical entity - the "International 
Prototype Metre". (* Actually, even before, for a few years the metre 
was based on other properties, including being a 10,000-th of the 
half-meridian through Paris, but those didn't last long.)

Later, scientists managed to measure wavelengths at precisions that 
exceeded that of the Prototype Metre, and therefore it was decided in 
1960 to redefine the metre as the vacuum wavelength of a particular 
emission of krypton-86.

Some two decades later, both time and the vacuum speed of light could be 
measured even better than wavelengths, and the metre was redefined in 
1983 as the distance travelled by light in vacuum within a certain 
period of time.

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