Empirical and Molecular Formulas

 Tells us relative number of atoms of each element it contains  Example: H 2 O: 2 atoms of H per 1 atom of O  ALSO: H 2 O: 2 mol of H per 1 mol of O  Mole concept allows us to calculate the empirical formula

 Compound is 73.9% Hg and 26.1% Cl. Find empirical formula.  Assume 100 grams 73.9 g Hg x (1 mol Hg) = mol Hg g Hg 26.1 g Cl x (1 mol Cl) = mol Cl 35.5 g Cl mol Cl = mol Cl = 1.99 mol Cl = 2 = HgCl 2 mol Hg mol Hg = 1 mol 1

 Ascorbic acid: 40.92% C, 4.58% H, and %O  Always divide the larger numbers (C and H) of moles by the smallest number of moles (O)

 Have to find Empirical Formula first!!!  Whole number multiple = molecular weight empirical formula weight  Multiple each subscript of the empirical formula by the whole number multiple

 Empirical formula = C 3 H 4 O 3  1 st, find empirical formula weight:  3(12.0) + 4(1.0) + 3(16.0) = 88.0 amu  Given: experimentally determined molecular weight = 264 amu  W.N.M.= molecular weight = 264 = 3 empirical formula weight 88  Molecular Formula = C9H12O9

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