1. Chemistry Empirical and Molecular Formulas

2. Objectives n Students will be able to: n Determine the empirical formula of a compound n Determine the molecular formula of a compound

3. Two types of formulas Empirical Formula n Gives the formula of a compound in the simplest whole number ratio of atoms/moles in a compound Molecular Formula n “True Formula”—gives the actual ratio of atoms/moles in a compound

4. B. Empirical Formula C2H6C2H6 CH 3 reduce subscripts n Smallest whole number ratio of atoms in a compound

5. Determine the empirical formulas for each of the following molecular formulas. 1. C 8 H 18 …..______________ 2. H 2 O 2 ……______________ 3. Hg 2 Cl 2 …..______________ 4. C 3 H 6 …….______________ 5. Na 2 C 2 O 4..._____________ 6. H 2 O......________________ 7. C 4 H 8 ….._______________ 8. C 4 H 6 …..________________ 9. C 7 H 12 …._______________ 10. C 2 H 4 O 2 ….___________ C 2 H 9 HO HgCl CH 2 NaCO 2 H 2 O CH 2 C 2 H 3 C 7 H 12 CH 2 O

6. 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.

7. B. Empirical Formula n Find the empirical formula for a sample of 25.9% N and 74.1% O. 25.9 g 1 mol 14.0 g = 1.85 mol N 74.1 g 1 mol 16.0 g = 4.63 mol O 1.85 mol = 1 N = 2.5 O

8. B. Empirical Formula N 1 O 2.5 Need to make the subscripts whole numbers  multiply by 2 N2O5N2O5

9. Steps for finding Molecular Formul a 1. Determine the empirical formula ( either given or you find it) 2. Find the molar mass of the empirical formula 3. Divide the Molecular Formula mass (given in problem) by the EF mass to create a ratio (n). 4. Multiply the EF formula by the whole # ratio (n). n= MF mass molecular formula = (EF) n EF mass 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.

10. Finding the Molecular Formula The empirical formula for adipic acid is C 3 H 5 O 2. 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 C 3 H 5 O 2 3(12.0 g) + 5(1.0) + 2(16.0) = 73.0 g

11. Finding the Molecular Formula The empirical formula for adipic acid is C 3 H 5 O 2. 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

12. Finding the Molecular Formula The empirical formula for adipic acid is C 3 H 5 O 2. 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 Step 4. Multiply the empirical formula by this number to get the molecular formula. (C 3 H 5 O 2 ) x 2 =C 6 H 10 O 4