Avogadro's number vs. the Kofh Number

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One of Kofh2u's objections was that Avogadro's number doesn't take account of different proportions of isotopic molecules in a substance but I don't see how his scheme would obviate that.

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The atomic weight listed on periodic tables reflects the ratio of isotopes for a certain element as they are found in nature. For example, the ratio of hydrogen, deuterium, and tritium found in one mole of water is reflected in the slightly higher atomic weight of Hydrogen (normal H = 1.0000 while in the periodic table H = 1.0079). Avogadro's number doesn't directly take into account differing weights, but the weights in the periodic table do. Just as a counterexample, if I concentrated pure tritium from a normal pool of hydrogen, I would use the atomic weight for tritium (3.0 I think) instead of 1.0 for hydrogen for calculating mass in one mole of tritium. The adjustment is in the atomic weight used, not Avo's number.So, I guess the answer is this. As long as you are using natural ratios of isotopes, the masses listed in the periodic table are sufficient, and one molar mass of any substance will still contain 6.022 x 10²³ molecules.

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Stoichiometrically speaking of coarse, since the spin of an atom can be calculated nowadays, there is no excuse not to hold molecular weight out to 4 or 5 decimal places.

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Depends on the precision needed. In my everyday work, 50 mM or 50.00323 mM is really the same thing when making biological buffers. Even when using isotope tagged biomolecules, we still use the common molecular weight (given a MW of > 50). It is called a fudge factor for a reason, it can be ignored if it doesn't fudge everything up. I'm not trying to downplay current knowledge, it is pretty cool that we can measure atomic mass with such precision.