1) The two values don't measure exactly the same thing. The mole is a unit for the amount of of material, while the kilogram is the unit for mass. From a che

1) The two values don't measure exactly the same thing. The mole is a unit for the amount of of material, while the kilogram is the unit for mass. From a chemistry perspective, the difference probably doesn't matter. If you have 1 mol of carbon 12, you know that it's exactly 12g. On the other hand, what if you had 1 mol of electrons? The conversion between the two quantities only works because we can use the isotope number as a conversion factor. 2) We don't exactly know Avogadro's constant. We've measured it fairly accurately, but, for your proposal to work, we'd be forced to define all 23 digits. We can then declare that 12g of carbon 12 is exactly 12g, but how do you know that you have exactly 12g of carbon twelve? Just from basic counting statistics, you'll probably be off by a few hundred billion atoms, and that's assuming that you have someone is and count all 6.02 * 10^23 atoms individually. The result is that the mass of the kilogram is now a derived and empirical value, adding uncertainty to every measurement of mass in every experiment. It's far preferable to leave the kilogram as an exactly defined unit and put the uncertainty into Avogadro's number, where it will only crop up when someone needs to make the conversion.