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A mole of a substance is the mass, in grams, that is numerically equal to that substance's molecular weight. The number of particles in one mole of a substance is called Avogadro's number. For example, since the molecular weight of water (H2O) is 18.01528 (the atomic weight of two atoms of hydrogen plus the atomic weight of one atom of oxygen), the mass of one mole of water is 18.01528 grams and contains an Avogadro's number of molecules.