7 3 NOTES Molecular Formulas E Molecular formula

7. 3 – NOTES Molecular Formulas

• E. Molecular formula • Since more than 1 compound could have the same empirical formula, in order to determine the molecular formula we need more information, such as the molar mass. • Molecular formula: the actual whole number ratio of atoms • For example, benzene has the molecular formula C 6 H 6, but the empirical formula is CH.

• The process is the same as empirical formula with one additional step. • Divide the molar mass given by the mass of the empirical formula. • Multiply this factor by the numbers in the empirical formula.

• Examples: • 1. 82. 7% of a compound is carbon and 17. 3% is hydrogen. If the molar mass is 58. 12 g, what is the molecular formula? • • C – 82. 7 g (1 mol/ 12. 0 g) = 6. 8916 / 6. 8916 = 1 x 2 = 2 • • H – 17. 3 g (1 mol/ 1. 01 g) = 17. 128/6. 8916 = 2. 5 x 2 = 5 • • C 2 H 5 = mass of 29. 1 • • 58. 12/ 29. 1 = 2 C 2 H 5 x 2 = C 4 H 10 •

• 2. Caffeine has the composition: 49. 5% carbon, 5. 19% hydrogen, 28. 9% nitrogen and 16. 5% oxygen. If the molar mass of caffeine is 194 g, what is the molecular formula? • • C – 49. 5 g (1 mol/ 12. 0 g) = 4. 125/ 1. 0312 = 4 • • H – 5. 19 g (1 mol/ 1. 01 g) = 5. 1386/ 1. 0312 = 5 • • N – 28. 9 g (1 mol/ 14. 0 g) = 2. 0642/ 1. 0312 = 2 • • O – 16. 5 g (1 mol/ 16. 0 g) = 1. 0312 / 1. 0312 = 1 • • C 4 H 5 N 2 O = 97. 1 • • 194/ 97. 1 = 2 C 4 H 5 N 2 O x 2 = C 8 H 10 N 4 O 2