1. % Composition, Empirical Formulas, & Molecular Formulas Chapter 9 sections 3 & 4

2. % Composition % = (part / whole ) x 100 % = (part / whole ) x 100 When calculating the % composition, you are calculating the % of each element in a compound When calculating the % composition, you are calculating the % of each element in a compound

3. % Composition Calculate the % Composition of MgO Calculate the % Composition of MgO Mg = 24.31g O = 16.00g Total = 40.31g / 40.31 X 100 = 60.31 % Mg = 39.69% O

4. % Composition Calculate the % Composition of iron (III) oxide Calculate the % Composition of iron (III) oxide % Fe = 69.94% % Fe = 69.94% % O = 30.06% % O = 30.06%

5. Empirical & Molecular Formulas Empirical formula – the smallest whole number ratio of elements Empirical formula – the smallest whole number ratio of elements Molecular formula – the true number of elements in a compound Molecular formula – the true number of elements in a compound

6. Empirical Formula What is the empirical formula for H 2 O 2 ? What is the empirical formula for H 2 O 2 ? HO HO What is the empirical formula for C 6 H 12 O 6 ? What is the empirical formula for C 6 H 12 O 6 ? CH 2 O CH 2 O

7. Steps for Calculating the Empirical Formula 1. List your givens 2. Change % to grams 3. Change grams to moles 4. Divide everything by the smallest number of moles 5. Write your formula

8. Empirical Formula Problem Calculate the empirical formula of a compound containing 40.05 % S and 59.95 % O. Calculate the empirical formula of a compound containing 40.05 % S and 59.95 % O. 40.05 g S x 59.95 g O x 1 mol S = 32.07 g S 1 mol O = 16.00 g O 1.249 mol 3.747 mol //// 1.249 mol = 1 1.249 mol = 3 SO 3

9. Empirical Formula Problem Calculate the empirical formula for a compound containing 48.64 g C, 8.16 g H, and 43.20 g O. Calculate the empirical formula for a compound containing 48.64 g C, 8.16 g H, and 43.20 g O. C 3 H 6 O 2 C 3 H 6 O 2

10. Steps for Calculating Molecular Formula 1. Calculate the empirical formula 2. Get the molecular mass of the empirical formula that you just determined 3. Divide the experimentally determined molecular mass (given) by the molecular mass of the empirical formula 4. You will get a whole number 5. Multiply everything in the empirical formula by this number

11. Molecular Formula Problem Calculate the molecular formula of a compound containing 40.68%C, 5.08%H, and 54.25%O with an experimentally determined molecular weight of 118.1 g/mol Calculate the molecular formula of a compound containing 40.68%C, 5.08%H, and 54.25%O with an experimentally determined molecular weight of 118.1 g/mol

12. Molecular Formula Problem 40.68g C x 1 mol C = 3.387 / 3.387 = (1)2 = 2 12.01 g C 12.01 g C 5.08g H x 1 mol H = 5.04 / 3.387 = (1.5)2= 3 1.01 g H 1.01 g H 54.25 g Ox 1 mol O = 3.390 / 3.387 = (1)2 = 2 16.00 g O 16.00 g O C2H3O2C2H3O2C2H3O2C2H3O2

13. Molecular Formula Problem C 2 H 3 O 2 C 2 H 3 O 2 Molecular Mass = 59.04 Molecular Mass = 59.04 EDMM / EFMM = 118.1 / 59.04 = 2 EDMM / EFMM = 118.1 / 59.04 = 2 Molecular Formula Molecular Formula C 4 H 6 O 4 C 4 H 6 O 4

14. Molecular Formula Problem Calculate the molecular formula of a compound containing 57.84 g C, 3.64 g H, and 38.52 g O with an experimentally determined molecular mass of 249.21 g/mol Calculate the molecular formula of a compound containing 57.84 g C, 3.64 g H, and 38.52 g O with an experimentally determined molecular mass of 249.21 g/mol C 12 H 9 O 6 C 12 H 9 O 6