Unit 7 Chemical Quantities Determining the Empirical Formula
- Slides: 6
Unit 7: Chemical Quantities Determining the Empirical Formula from Percent Composition
After today, you will be able to… • Calculate the empirical formulas for various compounds given their percent makeup
Empirical Formula Determination from % Composition To determine empirical formulas from % composition follow these steps: 1. Assume 100. 0 g of the compound (change % into grams) 2. Convert g moles for each element using molar mass 3. Divide each mole answer by the smallest mole answer 4. If you do not have a whole number, multiply by the smallest whole # to get a whole # ratio. 5. Skip to this step if you already have a whole number ratio from #3. These whole #s are the subscripts for each element.
To sum up these steps here is a poem for my favorite little chemists: “Percent to mass Mass to Moles Divide by small Multiply by whole!”
Example: What is the empirical formula for a compound that is 21. 2% N, 6. 10% H, 24. 3% S, and 48. 4%O? 21. 2 g. N x 1 mol N 14. 01 g. N 6. 10 g. H x 1 mol H 1. 01 g. H 24. 3 g. S x 1 mol S 32. 07 g. S 48. 4 g. O x 1 mol O 16. 00 g. O 1. 51 = 0. 758 = 6. 04 0. 758 = 3. 03 0. 758 = 2 = 7. 97~ 8 = 1 = 4 N 2 H 8 SO 4
Example: What is the empirical formula for a compound that is 69. 8% Fe and 30. 2% O? 69. 8 g. Fe x 1 mol Fe = 1. 25 = 1 (2) 55. 85 g. Fe 1. 25 30. 92 g. O x 1 mol O = 1. 89 1. 25 16. 00 g. O Fe 2 O 3 = 2 = 1. 5 (2) = 3
