Le Grand K is dead! Long live Le Grand K!
An old artifact kept in a vault outside Paris is no longer the standard kilogram
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Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram
#112For the next revision: remove the mole from the SI. It's completely pointless. A number is not a unit! Instead replace it by the bit, the fundamental unit of information.
The reason the mole is in the SI puzzled me too. It seems that the reason is that chemists don't care how many atoms there are in their reactions. It would be inconvenient: fractions of atoms don't make sense, and the numbers are far from human scale. They just want an agreement, so that even if they don't know the exact number of atoms in their test tube, they know they all have the same number. That's what units ar…
The point of the mole unit is this: 1 mole of nucleons (protons or neutrons) has a mass of 1 gram. So while a mole is "just" a name for a large number (akin to billion or quintillion), the actual underlying exact count isn't very important. Instead what matters is making the stochiometric computations easy to do from first principles; given the formula of a molecule, and the atomic weights of each element, you can compute the mass of one mole, and dividing the mass of your stuff tells you how many moles without any other conversion factor. If we used a different unit of mass than grams, such as pounds, then it would make sense to use a similarly different definition of mole to avoid throwing extra units into the equation.
The difficulty with this simplistic view is that it turns out that nucleons actually differ in mass slightly, depending on which nucleus they're in (and, also, isotopic ratios matter), which means there are a few different definitions you can use for the number of nucleons in 1 gram of stuff. In practice, the difference is small enough that it doesn't matter for most uses, especially if you build it into your table of atomic weights.
Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram
#113The new reference kilogram is an engineering marvel, but I’ve always wondered this: why didn’t they just build a reference gram instead? Seems like that would be at least a thousand times easier to pull off.
Basically, it's part of the lie of the mythology of the metric system.
The metric system did some great things (eliminated the use of measures that depended on the substance being measured, dry pints vs wet pints, bushel of wheat versus bushel of oats; simple relationship between units of length, area and volume), some things that were already common at the time (eliminated regional definitions of units; related the volume and mass of water), and some stupid things (metric prefixes).
But at the end of the day, the size of the meter and kilogram were chosen to be very nearly 3 Parisian feet and 2 Parisian pounds, because that made it easier to adopt.
Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram
#114Definition of mole changes too. One mole contains exactly 6.022 140 76×10²³ elementary entities (as opposed to the number of atoms in 12 grams of ¹²C). Effective 20 May 2019.
Eh? Straight 6.022*10^23 is what I learned in highschool chemistry in 2003..
Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram
#115The new reference kilogram is an engineering marvel, but I’ve always wondered this: why didn’t they just build a reference gram instead? Seems like that would be at least a thousand times easier to pull off.
The reference kilogram is estimated to have lost 50 micrograms over it's lifetime. I think it's safe to say that a reference gram would have lost a similar amount of mass. This would have caused a larger change in the definition of a gram/kilogram over time because the ratio of mass lost in the reference would have been greater.
Not much, but still.
Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram
#116Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram
#117Earlier quoted context omitted.
If you were emperor of the world, I would implore you to finance a multibillion dollar project to measure the gravitational constant to higher percision so that we could move to units that actually make sense. https://en.wikipedia.org/wiki/Planck_units
Came here to mention the Planck units, aka God's units :) Sadly they are a little outside of the human realm of experience, both in their single units (waaay to small usually) and in the units that the list contains (speed, electrical charge, etc; where we like "introductory" units such as length, time, weight)
With the exception of the Planck mass, which ends up being about that of a gnat.
Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram
#118The new reference kilogram is an engineering marvel, but I’ve always wondered this: why didn’t they just build a reference gram instead? Seems like that would be at least a thousand times easier to pull off.
I love that everyone took this as a serious comment.
Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram
#119Definition of mole changes too. One mole contains exactly 6.022 140 76×10²³ elementary entities (as opposed to the number of atoms in 12 grams of ¹²C). Effective 20 May 2019.
Eh? Straight 6.022*10^23 is what I learned in highschool chemistry in 2003..
It's being slightly tweaked:
https://chemistry.stackexchange.com/questions/91692/avogadro...
Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram
#120The new reference kilogram is an engineering marvel, but I’ve always wondered this: why didn’t they just build a reference gram instead? Seems like that would be at least a thousand times easier to pull off.
Basically, it's part of the lie of the mythology of the metric system. The metric system did some great things (eliminated the use of measures that depended on the substance being measured, dry pints vs wet pints, bushel of wheat versus bushel of oats; simple relationship between units of length, area and volume), some things that were already common at the time (eliminated regional definitions of units; related the…
This reply doesn't seem to address the comment it's replying to at all...