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Chemistry Empirical and Molecular Formulas Objectives Students will be able to: Determine the empirical formula of a compound Determine the molecular formula of a compound Two types of formulas Empirical Formula Gives the formula of a compound in the simplest whole number ratio of atoms/moles in a compound Molecular Formula “True Formula”—gives the actual ratio of atoms/moles in a compound B. Empirical Formula Smallest whole number ratio of atoms in a compound C 2H 6 reduce subscripts CH3 Determine the empirical formulas for each of the following molecular formulas. 1. C8H18…..______________ • C2H9 2. H2O2……______________ 3. Hg2Cl2…..______________ 4. C3H6…….______________ 5. Na2C2O4..._____________ 6. H2O......________________ 7. C4H8….._______________ 8. C4H6…..________________ 9. C7H12…._______________ 10. C2H4O2….___________ • • • • • • • • • HO HgCl CH2 NaCO2 H2O CH2 C2H3 C7H12 CH2O Finding Empirical Formula 1. Use the % of each element as its mass in the compound (ex 36.8% N, 63.2%O) 2. Convert mass of each element to moles. 3. Divide moles by the smallest # of moles to find subscripts. 4. If #’s are not whole #’s, multiply subscripts by 2, 3, or 4 to get whole #’s. B. Empirical Formula Find the empirical formula for a sample of 25.9% N and 74.1% O. 25.9 g 1 mol = 1.85 mol N =1N 1.85 mol 14.0 g 74.1 g 1 mol = 4.63 mol O = 2.5 O 16.0 g 1.85 mol B. Empirical Formula N1O2.5 Need to make the subscripts whole numbers multiply by 2 N2O5 Molecular formula: can be the same as the empirical formula or a Whole # multiple of the empirical. It is the true number of atoms of each element in a compound. Steps for finding Molecular Formula 1. 2. 3. 4. Determine the empirical formula(either given or you find it) Find the molar mass of the empirical formula Divide the Molecular Formula mass (given in problem) by the EF mass to create a ratio (n). Multiply the EF formula by the whole # ratio (n). n= MFmass molecular formula = (EF)n EFmass Finding the Molecular Formula The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid? Step 1 (given in problem) Determine Empirical Formula Step 2. Find the Molar mass of C3H5O2 3(12.0 g) + 5(1.0) + 2(16.0) = 73.0 g Finding the Molecular Formula The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid? 3(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g 146 2 73 Finding the Molecular Formula The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid? Step 4. Multiply the empirical formula by this number to get the molecular formula. 3(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g 146 2 73 (C3H5O2) x 2 = C6H10O4