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Can somebody (preferrably, a physicist or a chemist) explain to me two things. 1. Why do we need both kilogram and mole as fundamental units when they measure

by zvrba 15y ago

Can somebody (preferrably, a physicist or a chemist) explain to me two things. 1. Why do we need both kilogram and mole as fundamental units when they measure essentially the same thing (at least in case when the thing being measured is pure)? 2. Why not reverse the definition: postulate that 1 mole is Avogadro's number of elementary particles; 1 mole of C-12 has 12 grams and derive (kilo)gram from that.

vilhelm_s 15y ago
(I'm not a chemist, but): 1. They are not both fundamental, a mole is defined as the amount of substance that contains an equal number of elementary entities as there are atoms in 0.012kg of the isotope carbon-12. So the mole is defined in terms of the kg. 2. This is essentially Arnold Nicolaus' project (described in the article). He wants to define the kg as the mass of a certain number of atoms. But we don't yet have an accurate way of counting atoms, so if we pick a number (i.e. define Avogadro's number by fiat) today, then later when we count more carefully the mass of everything (denoted in kg) will fluctuate.
rprospero 15y ago
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.
deleted 15y ago
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by zvrba 15y ago

vilhelm_s 15y ago

(I'm not a chemist, but): 1. They are not both fundamental, a mole is defined as the amount of substance that contains an equal number of elementary entities as there are atoms in 0.012kg of the isotope carbon-12. So the mole is defined in terms of the kg. 2. This is essentially Arnold Nicolaus' project (described in the article). He wants to define the kg as the mass of a certain number of atoms. But we don't yet have an accurate way of counting atoms, so if we pick a number (i.e. define Avogadro's number by fiat) today, then later when we count more carefully the mass of everything (denoted in kg) will fluctuate.

rprospero 15y ago

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.

deleted 15y ago

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