Happy Mole Day!
Published on Oct 23, 2007 at 11:58 am.
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My chemistry friends tell me that this is National Mole Day. A mole is the amount of a subestance equal to one Avogadro’s number of molecules or atoms of that substance. Avogadro’s number is about 6.0221418×1023. Almost universally in textbooks this is truncated to three significant figures for solving homework problems, so students learn Avogadro’s number as 6.02×1023. The American way of writing dates is to put the month first, and then the date, so this is October 23 rather than 23 October. Thus, we would write the date as 10/23. Mole Day starts at 6:02 am, so that would be 6:02 10/23, very reminiscent of Avogadro’s number. National Mole Day is supposed to promote chemistry awareness.
 Interestingly, Amedeo Avogadro was a physicist rather than a chemist. He realized that the volume of an ideal gas at a given pressure depends upon the number of molecules in the gas. He published this hypothesis in 1811. In theory, if you knew the number of molecules in a given volume of an ideal gas at a particular pressure, then you could figure out the pressure at other volumes, or the volume of different numbers of molecules at any pressure. Avogadro himself, though, if he worked on the matter further, never published any numeric values associated with this hypothesis. However, Johann Loschmidt did try to use the kinetic theory of gasses compute the number of molecules per unit volume of an ideal gas at standard temperature and pressure. Over the years, various attempts to compute Avogadro’s constant, but different computations gave different results because there was little agreement on just what the conditions should be to compute a number. A mole of a substance is one Avogadro’s number worth of its molecules. But, if there was no standardized way of computing Avogadro’s number, that meant no standardized way of computing a mole.Â
 Eventually, though, by agreement, it was decided that a mole is equal to the amount of a substance that has as many elemental units (such as molecules or atoms) as exactly 12 grams of carbon. The only problem is that carbon, like all other atoms, comes in multiple isotopes. An isotope of an element is an atom containing the same number of protons but different number of neutrons. That means that different isotopes have different masses.  When you get together a mass of carbon, most of that carbon is one particular isotope, that having atomic weight 12, but there are other isotopes present as well, having atomic weights 13 and 14. Varying the ratios of these isotopes would change the number of atoms in 12 grams of the substance. So, chemists and physicists define a mole as the theoretical number of atoms in exactly 12 grams of perfectly pure 12C, which is the most common isotope. That number was named for Amedeo Avogadro for his work in pioneering studies in the area, even though he himself never computed a numerical value for his hypothesis (or, at least, never published a value).  Avogadro’s number is then defined as being that theoretical number of carbon atoms in one mole of carbon-12.
 Astroprof     Â
