In 1811, Amedeo Avogadro theorized that "equal volumes of all gases at the same temperature and pressure contain the same number of molecules." This is now known as Avogadro's Law. Experiments based on this principle eventually led to the calculation of the number of molecules in one cubic centimeter of gas.This number of molecules were determined to be roughly 6.02 x 10^23; this was called "Avogadro's number." The element Sodium has the number 6.02 x 10^23 which has been …
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…the number of molecules in a mole as 6.004 x 10^23 in 1924. A variation of this experiment is sometimes done today by beginning chemistry students. Modern methods of determining Avogadro's number is used on x-ray crystallography to get precise dimensions in crystals. These can produce extremely direct values of Sodium with an error of less than 0.000 000 01. The National Institute of Standards and Technology is an area where more information about physical constants and standards can be located.