First, in 1811 in Italy, Amadeo Avogadro hypothesised that equal volumes of different gases at the same tempertature contain equal numbers of molecules. Using that information, Stanislao Cannizzaro used developed a set of atomic weights for the known elements by comparing the masses of equal volumes of gas. Later, Johann Josef Loschmidt calculated the size of a molecule of air in 1865, and therefore was able to estimate the amount of moleciles in a given volume of air. All of these lead to the developement of Avogadro's number, or the mole. The word mole comes from the german word molekulargewicht, meaning molecular weight, and it was coined in 1901.
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http://www.chemistryexplained.com/Ma-Na/Mole-Concept.html
