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The Mole Atomic Mass • Relative weight - the atomic mass unit (amu) is used to describe the mass of an atom relative to a carbon-12 isotope. • 1mole of any element makes the relative weight equal to the gram weight. Element H O relative weight 1 amu 16 amu atomic weight 1 g/mol 16 g/mol The Mole • A mole is a quantity - 1 mole of any substance contains 6.02 x 1023 particles – Example: – 1 dozen = 12 things – 1 pair = 2 things – 1 ream = 500 things • Avogadros’s number = 6.02x1023 Molar Mass (MM) • The mass in grams of 1 mole of a molecule is the molar mass. • The mass in grams of an ionic compound is called the formula mass. • A formula unit is an ionic compound. Molar or Formula Mass Calculations Example: 1. What is the formula mass of CuO? Cu = 63.55 g/mol O = 16.00 g/mol CuO = 63.55 + 16.00 = 79.55 g/mol 2. What is the molar mass of H2O? H = 1.01 g/mol, O = 16.00 g/mol H2O = (1.01 x 2) + 16.00 = 18.02 g/mol Molar Mass • Formula mass and molar mass are calculated the same way. Formula mass refers to ionic compounds and molar mass refers to molecules. • The formula gives the mole ratio of each atom in the compound. Example: 1 mole of H2O = 2 mole of hydrogen atoms and 1 mole of oxygen atoms Mole Conversions • Balanced chemical equations give the mole ratio of reactants and products. – 2H2 + O2 2H2O : 2 moles of hydrogen reacts with 1 mole of oxygen to make 2 moles of water. • To find grams or molecules/formula units you must convert using moles. Converting Between Moles and grams 1. How many grams is 2 moles of water? Use the molar mass to convert between moles and grams: 2 moles H2O x 18.02 g = 36.04 g mole 2. How many moles are in 5.0 grams of H2O? 5 g H2O x mole = .28 moles 18.02 g Converting between Molecules and Moles 1. How many molecules are in 2 moles of water? Use Avogadro's number to convert from moles to molecules. 2 moles H2O x 6.02 x 1023 molecules = mole 1.20 x 1024 molecules