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Chemistry Chapter 11 Empirical and Molecular Formulas

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Empirical Formulas The simplest whole number ratio of the elements in a compound (not necessarily the actual formula for a compound) Ex –H 2 O 2 = HO –H 2 O= H 2 O –C 6 H 12 = CH 2 –C 8 H 16 = CH 2 Can be calculated from % composition

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Empirical Formula (Cheat Sheet) 1. % composition (if = 100%) converts to mass 2.Convert from mass to moles 3.Calculate simplest ratio (divide all # mol by the smallest # moles) 3 ½. If not all whole numbers, multiply all by a whole # to create whole #’s 4. Use simplest ratio as subscripts

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Examples 11.2 % Hydrogen and 88.8% Oxygen

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Examples 36.84% N and 63.16% O

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Examples 60% C, 4.44%H, and 35.56% Oxygen

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Molecular Formula Actual number of moles of each element in a compound Need more info than just % comp Need molar mass of the empirical formula Need an experimental mass of the actual compound

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Molecular Formula (Cheat Sheet) 1.Use steps to calculate empirical formula 2.Calculate molar mass of empirical formula 3.The question will give you experimental mass of the compound 4. Experimental mass. Empirical form. Molar mass = 5. Multiply subscripts in empirical by your answer (whole number)

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Example A colorless liquid is found to be composed of 46.68% N and 53.32% O and has a molar mass of g/mol. What is the molecular formula?

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