1 Empirical & Molecular Formulas Chapter 3 Percent Composition % Element = atomic mass of element x # atoms x 100% in compoundmolar mass of the compound Calculate the percent phosphorus in calcium phosphateEmpirical 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 = C8H18 Empirical formula = C4H9 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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