Chapter 3 The Mole The Mole: The amount of a substance that contains the same number of ?entities? as 12 grams of Carbon-12 (C-12) 1 mol C-12 = 6.022 x 1023 Carbon atoms 1 mol H20 = 6.022 x 1023 molecules H20 1 mol H20 = 2 mols H and 1 mol O (ratio never changes) Definitions The mole: the SI unit for amount of substance Specifies the # of objects in a fixed mass of substance Avogadro's #: 6.022 x 1023 All isotopes are considered to have the same atomic mass The mass on the periodic table is an average of the masses of the naturally occurring isotopes The atomic mass of an element expressed in amu is numerically the same as the mass of 1 mole of atoms of the element expressed in grams Example: