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Unit: Chemical Quantities Determining the Molecular Formula from the Empirical Formula After today, you will be able to… • Calculate the empirical formulas for various compounds given their percent makeup Molecular Formula Determination from the Empirical Formula To determine molecular formulas from the empirical formula follow these steps: 1. First determine the empirical formula 2. Use the formula: Molar mass of molecular formula Molar mass of empirical formula 3. Multiply the subscripts in the empirical formula by this whole number to get the molecular formula Example: What is the molecular formula for a compound that is 74.0% C, 8.70% H, and 17.3% N and has a molar mass of 243.39g? 70.4gC x 1 mol C 6.16 Empirical: C5H7N = = 5 12.01gC 1.23 5C=12.01(5) = 60.05 7H=1.01(7) = 7.07 8.70gH x 1 mol H = 8.61 = 7 1N=14.01(1) = +14.01 1.01gH 1.23 81.07 17.3gN x 1 mol N = 1.23 = 1 14.01gN 1.23 243.39 81.07 = 3 x C5H7N C15H21N3 Time to Practice! Complete WS 7