Name Date Pd Chemistry – Unit 5 Worksheet 3 Empirical and Molecular Formulas Show all your work when solving the following problems. Be sure to include units and label your answer. 1. Find the empirical formula of a compound containing 32.0 g of bromine and 4.9 g of magnesium. 32.0 g 1 mole = 0.401 mole Br 79.9 g 1:2 ratio 4.9 g 1 mole = 0.20 mole Mg 24.31 g MgBr2 2. What is the empirical formula of a carbon-oxygen compound, given that a 95.2 g sample of the compound contains 40.8 g of carbon and the rest oxygen? 95.2 g – 40.8 g = 54.4 g O 40.8 g 54.4 g x 1 mole = 3.40 moles C 12.0 g 1:1 ratio 1 mole = 3.40 moles O 16.0g CO 3. A compound was analyzed and found to contain 9.8 g of nitrogen, 0.70 g of hydrogen, and 33.6 g of oxygen. What is the empirical formula of the compound? 9.8 g 1 mole = 0.70 moles N 14.0g 1:1:3 ratio 0.70 g 1 mole = 0 .69 moles H 1.01 g 33.6 g 1 mole = 2.10 moles O 16.0 g HNO3 4. A compound composed of hydrogen and oxygen is found to contain 0.59 g of hydrogen and 9.40 g of oxygen. The molar mass of this compound is 34.0 g/mol. Find the empirical and molecular formulas. 0.59 g 1 mole = 0.58 mole H 1:1 ratio; empirical formula: HO 1.01 g empirical formula mass: 17.0 g/mol molecular formula mass: 34.0 g/mol 9.40 g 1 mole = 0.588 mole O mfm is 2x efm so the molecular formula 16.0 g is: H2O2 ©Modeling Instruction–AMTA 1 U5 ws 2 v3.0 5. A sample of iron oxide was found to contain 1.116 g of iron and 0.480 g of oxygen. Its molar mass is roughly 5 x as great as that of oxygen gas. Find the empirical formula and the molecular formula of this compound. 1.116 g .480 g 1 mole = 0.0200 mole Fe 55.8 g Since the efm and the mfm are nearly identical, the ef and the mf must also be identical: Fe2O3 1 mole = 0.0300 mole O 16.0 g efm: 2(55.8) + 3(16.0) = 159.6 g/mol mfm: 32.0 g O2 1 mole 5 = 160. g/mol 6. Find the percentage composition of a compound that contains 17.6 g of iron and 10.3 g of sulfur. The total mass of the compound is 27.9 g. 17.6 g Fe = 0.631 = 63.1 % Fe 27.9 g 10.3 g S = 0.369 = 36.9 % S 27.9 g 7. Find the percentage composition of a compound that contains 1.94 g of carbon, 0.48 g of hydrogen, and 2.58 g of sulfur in a 5.00 g sample of the compound. 1.94 g C = 0.388 = 38.8 % C 5.00 g 0.48 g H = 0.096 = 9.6 % H 5.00 g 2.58 g S = 0.516 = 51.6 % S 5.00 g 8. 16.0 What is the % by mass of oxygen in Mg(NO3)2 ? 6 = 96.0 g 24.3 + 2(14.0) + 6(16.0) = 148.3 g 96.0 g = 0.647 = 64.7 % O 148.3 g ©Modeling Instruction–AMTA 2 U5 ws 2 v3.0