1. Chapter 9.6 Finding the formula of a Compound

7. Deriving the Formula Step 1: List the mass of the element. Element Mg O Mass (from experiment) 1.20 g 0.80 g Relative atomic mass 24 16 Number of moles Molar ratio (divide by the smallest number from the previous row) 1.20 24 = 0.05 0.05 0.05 = 1 0.05 0.05 = 1 0.80 16 = 0.05

8. Deriving the Formula Step 2: State the A r of the element. Molar ratio (divide by the smallest number from the previous row) Number of moles 16 24 Relative atomic mass 0.80 g 1.20 g Mass (from experiment) O Mg Element 1.20 24 = 0.05 0.05 0.05 = 1 0.05 0.05 = 1 0.80 16 = 0.05

9. Deriving the Formula Step 3: Derive the number of moles by dividing the mass with A r . Molar ratio (divide by the smallest number from the previous row) Number of moles 16 24 Relative atomic mass 0.80 g 1.20 g Mass (from experiment) O Mg Element 1.20 24 = 0.05 0.05 0.05 = 1 0.05 0.05 = 1 0.80 16 = 0.05

10. Deriving the Formula Step 4: Obtain the molar ratio. The empirical formula is MgO . Molar ratio (divide by the smallest number from the previous row) Number of moles 16 24 Relative atomic mass 0.80 g 1.20 g Mass (from experiment) O Mg Element 1.20 24 = 0.05 0.05 0.05 = 1 0.05 0.05 = 1 0.80 16 = 0.05

12. Worked Example 1 A sample of an oxide of copper contains 8g of copper combined with 1g of oxygen. Find the empirical formula of the compound . Empirical formula of the compound is Cu 2 O 1 or usually written as this Cu 2 O. Element Cu O Step 1 Mass of element 8 1 Step 2: A r 64 16 Step 3: No. of Moles (Mass/ A r ) 8 = 0.125 64 1 = 0.0625 16 Step 4: Molar ratio (Divide by smallest number) 0.125 = 2 0.0625 0.0625 = 1 0.0625

14. Molecular Formula

20. Example 3 Propane has the empirical formula CH 2 . The relative molecular mass of propane is 42. Find the molecular formula of propane n = relative molecular mass of the compound Mr of the empirical formula n = 42 (12 X 1) + 2 = 3 Molecular formular = (Empirical formula ) n = (CH 2 ) 3 Hence the molecular formula for propane is C 3 H 6 = C 3 H 6

21. Example 4 Compound X contains 40.0% carbon, 6.6% hydrogen and 53.3% oxygen. Its relative molecular mass is 180. What is the molecular formula of X? Molar ratio Number of moles 16 1 12 Relative atomic mass 53.3 6.6 40.0 Percentage in compound O H C Element = 3.3 12 40.0 = 3.3 16 53.3 = 1 16 3.52 = 2 16 3.52 = 6.6 1 6.6 = 1 3.3 3.3

22. Example 2 (continued) Compound X contains 40.0% carbon, 6.6% hydrogen and 53.3% oxygen. Its relative molecular mass is 180. What is the molecular formula of X? The empirical formula of X is CH 2 O . = M r from empirical formula Relative molecular mass 180 30 = 6 Hence, the molecular formula of X = (CH 2 O) 6 = C 6 H 12 O 6 M r of CH 2 O = 12 + (2 x 1) + 16 = 30 n =