Percent Composition, Empirical Formula and Molecular Formula

Percent Composition 1 pizza 2 slices pineapple 6 slices pepperoni 4 slices cheese What % of the pizza slices are pineapple? pepperoni? just cheese? (2/12) x 100 = 17% pineapple, (6/12) x 100 = 50% pepperoni, (4/12) x 100 = 33% cheese

Percent Composition of a Compound from experimental data ex: a lab sample contains g carbon and g oxygen. compound mass = g g = g % C: (24.02 g / g) x 100 = 27.29% C % O: (64.00 g / g) x 100 = % O good because 27.29% % = 100%

Percent Composition of a Compound from a chemical formula ex: find the percent composition of CO 2. MM of C = g/mole MM of O = g/mole molar mass of compound = (16.00) = g/mole % C: (12.01 g / g) x 100 = 27.29% % O: (2X16.00 g / g) x 100 = 72.71% good because 27.29% % = 100%

Empirical Formula Empirical formula is smallest whole number ratios of elements in a compound. If an unknown sample is analyzed and the amount of each element is determined, an “elementary” formula can be created. smallest, whole-number ratio of moles of each element in the compound

Calculating Empirical Formula 1.If given %, assume 100g sample. 2.Convert mass of each element to moles. 3.Determine the smallest value from step 2 and divide all answers by that value. 4.Are your answers whole numbers or close enough to round? If so, they become the whole number subscripts. If not, go to next step 5.If necessary, multiply by the smallest integer possible to make each a whole number. 6.These whole numbers are the subscripts in the empirical formula (also called mole ratio)

Sample Problem Determine the empirical formula of a compound that is 40% carbon, 6.7 % hydrogen and 53.3 % oxygen. 40%  40g x 1 mole = g 6.7 %  6.7 g x 1 mole = g 53.3%  53.3 g x 1 mole = g 3.3/3.3 = 1 6.7/3.3 = 2 3.3/3.3 =1 Final Answer CH 2 O

Sample Problem 2: Ibuprofen, the active ingredient in anti-inflammatory medications like Advil, is 75.69% carbon, 8.8% Hydrogen and % oxygen. Determine the empirical formula %  75.69g x 1 mole = g 8.8 %  8.8 g x 1 mole = g 15.51%  g x 1 mole = g 6.3/.97 = /.97 = /.97 = 1 These answers are NOT whole numbers and not close enough to round, so you must multiply them by an integer that will make them whole numbers. In this case, multiply by 2 Final answer: C 13 H 18 O 2

Molecular Formula Actual number of atoms of each element in a molecule or formula unit. hydrogen peroxide = H 2 O 2 experimental molar mass needs to be provided.

Calculating Molecular Formula 1.Find molar mass of empirical formula. 2.Divide given molecular molar mass by molar mass of empirical formula. 1.Has to equal a whole number “n” 2.Multiply each subscript by n.

Model Problem 1 What is the empirical formula of a molecule containing 65.5% carbon, 5.5% hydrogen and 29.0% oxygen? C3H3OC3H3O If the molar mass of the compound is 110 grams per mole, what is the molecular formula? Mass of empirical formula is /55 = 2 multiply subscripts by 2 and get C 6 H 6 O 2 as your answer

Model Problem 2 What is the empirical formula of a molecule containing 18.7 % lithium, 16.3 % carbon and 65.0 % oxygen? Li 2 CO 3 If the gfm of the compound is 73.8 grams per mole, what is the molecular formula? Empirical formula mass = 73.8, therefore the molecular formula is the same as the empirical. Also, Li 2 CO 3 is ionic, so the EF and MF are the same