Practice Test #2: Molar Mass, Percent Composition, Moles, Hydrates, and Empirical and Molecular Formulas Answers: 1. 2. 3. 4. 5. c a b d b 6. a 7. d 8. c 9. a 10. d 1. Theobromine is a compound found in cocoa, the main ingredient in chocolate. Its formula is C7H8N2O3. What is the percentage of nitrogen found in theobromine? C 7·12.0 = 84.0 H 8·1.01 = 8.08 N 2·14.0 = 28.0 O 3·16.0 = 48.0 168.08 g/mol 28.0 ·100 = 16.66% N 168.08 2. If a sample of theobromine contains 2.27 x 1019 carbon atoms, how much will the sample weigh? 168 g 1 mole C7H8N2O3 C7H8N2O3 1 mole 6.022 x 1023 molecules C7H8N2O3 C7H8N2O3 1 molecule C7H8N2O3 2.27 x 1019 C atoms 7C atoms = 9.05 x 10-4 g C7H8N2O3 3. Chemical Compound Z contains 29.98% carbon, 6.73% hydrogen, 23.32% nitrogen, and 39.97% oxygen. The molar mass of compound Z is 480. g/mol. What are the empirical and molecular formulas of compound Z? 1 mol C 29.98 g C 12.01 g C 1 mol H 1.01 g H = 2.496 mol C = 1.5 C ·2 = 3 C 1.665 mol 6.73 g H = 6.66 mol H 1.665 mol 1 mol N 23.32 g N 14.01 g N = 1.665 mol N 1.665 mol = 4 H ·2 = 8 H = 1 N ·2 = 2 N 1 mol O 39.97 g O = 2.498 mol O = 1.5 O ·2 = 3 O 1.665 mol 16.00 g O Empirical formula is C3H8N2O3 MM = 120. g/mol 480. g/mol = 4 120. g/mol Molecular formula is C12H32N8O12 4. Laboratory experiments were conducted to determine the amount of water in samples of a hydrate of magnesium sulfate. The hydrate was heated gently. The anhydrous salt remained after the water was evaporated from the samle. The percentage of water in the sampleis 51.14%. What is the formula for the hydrate? magnesium sulfate, 51.14% MgSO4 ⋅ x H2O Mg 1 ⋅ 24.3 = 24.3 S 1 ⋅ 32.1 = 32.1 O 4 ⋅ 16.0 = 64.0 H2O x ⋅ 18.0 = 18.0x (120.4 + 18x) g/mol 18.0x ⋅ 100 = 51.14% (120.4+ 18x) 18x (120.4+ 18x) = 0.5114 18x = 0.5114(120.4+ 18x) 18x = 61.57 + 9.2052x 8.7948x = 61.57 x = 7 ... MgSO4 ⋅ 7 H2O magnesium sulfate heptahydrate 5. What is the molar mass of phosphoric acid? H3PO4 H 3·1.01 = 3.03 P 1·31.0 = 31.0 O 4·16.0 = 64.0 98.03 g/mol 6. What is the percentage of hydrogen in phosphoric acid? H3PO4 H 3·1.01 = 3.03 P 1·31.0 = 31.0 O 4·16.0 = 64.0 98.03 g/mol 3.03 98.03 ·100 = 3.09% H 7. How many hydrogen atoms are in a sample of phosphoric acid that weighs 11.1g? 3H atoms 1 molecule H3PO4 6.022 x 1023 molecules H3PO4 1 mole H3PO4 1 mole 98.0 g H3PO4 H3PO4 = 2.05 x 23 10 11.1 g H3PO4 H atoms 8. Octadiene contains 88.80% carbon and 11.20% hydrogen. If the molar mass of octadiene is 162 g/mol, what are the empirical and molecular fomulas? 1 mol C 88.80 g C 12.01 g C = 7.394 mol C 7.394 mol = 1 C ·2 = 2 C 1 mol H 11.20 g H = 11.11 mol H = 1.5 H ·2 = 3 H 7.394 mol 1.008 g H Empirical formula is C2H3 MM = 27. g/mol 162 g/mol = 6 27.0 g/mol Molecular formula is C12H18 9. What is the mass of a sample of technetium (VII) oxalate which contains 3.85 x 1040 oxygen atoms? 812 g 1 mole Tc2(C2O4)7 Tc2(C2O4)7 1 mole 6.022 x 1023 molecules Tc2(C2O4)7 Tc2(C2O4)7 1 molecule Tc2(C2O4)7 3.85 x 1040 O atoms 28 O atoms = 1.85 x 1018 g Tc2(C2O4)7 10. Alunogen is a hydrate of aluminum sulfate that contains 48.63% water. What is the coefficient of water in the hydrate? aluminum sulfate, 48.63% Al2(SO4)3 ⋅ x H2O Al 2 ⋅ 27.0 = 54.0 S 3 ⋅ 32.1 = 96.3 O 12 ⋅ 16.0 = 192 H2O x ⋅ 18.0 = 18.0x (342.3 + 18x) g/mol 18.0x ⋅ 100 = 48.63% (342.3+ 18x) 18x (342.3+ 18x) = 0.4863 18x = 0.4863(342.3+ 18x) 18x = 166.46 + 8.7534x 9.2466x = 166.46 x = 18 ... Al2(SO4)3 ⋅ 18 H2O