6.2H Empirical Formulas.doc Homework Review for Class Calculating an Empirical Formula Given the following information calculate the empirical formula for the molecule. 1. 56.33 % Oxygen, 43.67 % Phosphorus Solution:P2O5 STEP 1. Change the percent to grams If 100 grams of this compound were present there would be: Phosphorus 43.67 grams Oxygen 56.33 grams STEP 2. Calculate the number of moles of each element. You will need the molar mass for each element. Atom Mass 1 Moles Molar _ Mass 43.67grams 1mole Phosphorus 1.4 moles of P 1 30.97grams Oxygen 56.33grams 1mole 3.5 moles of O 1 16.00grams STEP 3. Calculate the Subscripts for the Empirical Formula. 1.4 moles of Phosphorus 3.5 moles of Oxygen DIVIDE ALL NUMBERS BY THE SMALLEST NUMBER 1.4 / 1.4 = 1.00 Phosphorus 3.5 / 1.4 = 2.5 Oxygen To adjust these numbers to whole numbers multiply both by 2 to get 2 and 5. The 2 will go with the Phosphorus and the 5 goes with the O. P2O5 2. 32.38% Sodium, 22.65 % Sulfur, 44.99% Oxygen Solution:Na2SO4 STEP 1. Change the percent to grams If 100 grams of this compound were present there would be: Sodium 32.38 grams Sulfur 22.65 grams Oxygen 44.99 grams STEP 2. Calculate the number of moles of each element. You will need the molar mass for each element. Atom Mass Moles 32.38grams 1mole Sodium 1.4 moles of Na 1 23.00grams1 22.65grams Molar1mole_ Mass Sulfur .7 moles of S 1 32.06grams Oxygen 44.99grams 2.8 moles of O 1 1mole 16.00grams STEP 3. Calculate the Subscripts for the Empirical Formula. 1.4 moles of Sodium .7 moles of Sulfur 2.8 moles of Oxygen DIVIDE ALL NUMBERS BY THE SMALLEST NUMBER 1.4 / .7 = 2.00 Sodium .7 / .7 = 1.00 Sulfur 2.8 / .7 = 4.0 Oxygen No adjustment is needed for these numbers, they work out on the first attempt and if you check the Polyatomic Ion chart SO4 is a common ion. Na2SO4 3. 56.6% Potassium, 8.7% Carbon, and 34.7% Oxygen Solution:K2CO3 STEP 1. Change the percent to grams If 100 grams of this compound were present there would be: Potassium 56.6 grams Carbon 8.7 grams Oxygen 34.7 grams STEP 2. Calculate the number of moles of each element. You will need the molar mass for each element. Atom Mass Moles 56.6grams 1mole Potassium 1.4 moles of K 1 39.10grams1 8.7grams Molar1mole_ Mass Carbon .7 moles of C 1 12.01grams Oxygen 34.7grams 2.2 moles of O 1 1mole 16.00grams STEP 3. Calculate the Subscripts for the Empirical Formula. 1.4 moles of Potassium .7 moles of Carbon 2.2 moles of Oxygen DIVIDE ALL NUMBERS BY THE SMALLEST NUMBER 1.4 / 1.7 = 2.00 Potassium .7 / .7 = 1.00 Carbon 2.2 / .7 = 3.1 Oxygen No adjustment is needed for these numbers, they work out on the first attempt and if you check the Polyatomic Ion chart CO3 is a common ion. K2CO3 4. 69.9% Iron, and 30.1% Oxygen Solution:Fe2O3 STEP 1. Change the percent to grams If 100 grams of this compound were present there would be: Iron 69.9 grams Oxygen 30.1 grams STEP 2. Calculate the number of moles of each element. You will need the molar mass for each element. Atom Mass Moles 69.9grams 1mole Iron 1.25 moles of Fe 1 55.85grams Oxygen 30.1grams 1 1.88 moles of O 1 Molar _ Mass STEP 3. Calculate the Subscripts for the Empirical Formula. 1.25 moles of Iron 1mole 1.88 moles of Oxygen 16.00grams DIVIDE ALL NUMBERS BY THE SMALLEST NUMBER 1.25 / 1.25 = 1.00 Iron 1.88 / 1.25 = 1.5 Oxygen To adjust these numbers to whole numbers multiply both by 2 to get 2 and 3. The 2 will go with the Iron and the 3 goes with the O. Fe2O3 .