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Lecture_5--Empirical_and_Molecular_Formulas

# Lecture_5--Empirical_and_Molecular_Formulas - Percent...

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1 Empirical & Molecular Formulas Chapter 3 Percent Composition % Element = atomic mass of element x # atoms x 100% in compound molar mass of the compound Calculate the percent phosphorus in calcium phosphate Empirical and Molecular Formulas Empirical Formula - the smallest whole number ratio of atoms (or moles of atoms) in a compound. Molecular Formula - the actual number of atoms in a molecule (multiple of the empirical formula). Example : Octane Molecular formula = C 8 H 18 Empirical formula = C 4 H 9 Sample Problem: Empirical & Molecular Formulas What is the molecular formula of the sugar ribose if it is 40.0% C, 6.7% H, and 53.3% O and it has a molecular mass of 150.1 g/mol? Sample Problem Solution First, determine the empirical formula: Step 1. Assume 100.0 g of the compound. If you have 100g of the material, you will have the following masses of C, H, & O: 40.0 g C 6.7 g H 53.3 g O Step 2. Convert grams to moles: 40.0 g C 1 mol C = 3.33 mol C 12.01 g C 6.7 g H 1 mol H = 6.6 mol H 1.01 g H 53.3 g O 1 mol O = 3.33 mol O 16.00 g O

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