Another definiton of the mole is the atomic or molecular mass of the substance expressed in grams, e.g. one mole of water will weigh 18.015 g. It was first defined by Amedeo Avogadro (pictured).
(Video titled "khanacademy - August 27, 2009 - Introduction to the idea of a mole as a number (vs. an animal)", sourced from http://www.youtube.com/watch?v=AsqEkF7hcII&feature=channel)
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