Local Chemistry Olympiad Answer Key

amount of substance ampere atmosphere atomic mass unit Avogadro constant Celsius temperature centi– prefix coulomb density electromotive force energy of activation enthalpy entropy equilibrium constant ABBREVIATIONS AND SYMBOLS Faraday constant F molar mass free energy G mole frequency ν Planck’s constant gas constant R pressure gram g rate constant hour h reaction quotient joule J second kelvin K speed of light kilo– prefix k temperature, K liter L time measure of pressure mm Hg vapor pressure milli– prefix m volt molal m volume molar M year n A atm u NA °C c C d E Ea H S K CONSTANTS M mol h P k Q s c T t VP V V y R = 8.314 J mol–1 K–1 R = 0.08314 L bar mol–1 K–1 F = 96,500 C mol–1 F = 96,500 J V–1 mol–1 NA = 6.022 × 1023 mol–1 h = 6.626 × 10–34 J s c = 2.998 × 108 m s–1 0 °C = 273.15 K 1 atm = 1.013 bar = 760 mm Hg Specific heat capacity of H2O = 4.184 J g –1 K –1 EQUATIONS E = Eo− 1 1A 1 H RT ln Q nF −∆H ln K =   R o 1    + constant T   k  1− 1 = Ea  ln  2  T1 T2 k1  R  PERIODIC TABLE OF THE ELEMENTS 18 3 Li 2 2A 4 Be 13 3A 5 B 14 4A 6 C 15 5A 7 N 6.941 9.012 10.81 12.01 14.01 1.008 11 Na 22.99 19 K 39.10 12 Mg 24.31 20 Ca 40.08 3 3B 21 Sc 44.96 4 4B 22 Ti 47.88 5 5B 23 V 6 6B 24 Cr 7 7B 25 Mn 50.94 52.00 54.94 8 8B 26 Fe 55.85 9 8B 27 Co 58.93 10 8B 28 Ni 58.69 11 1B 29 Cu 63.55 13 Al 12 2B 30 Zn 26.98 31 Ga 65.39 69.72 14 Si 28.09 32 Ge 72.61 15 P 30.97 33 As 74.92 16 6A 8 O 17 7A 9 F 16.00 19.00 16 S 32.07 34 Se 78.97 17 Cl 35.45 35 Br 79.90 8A 2 He 4.003 10 Ne 20.18 18 Ar 39.95 36 Kr 83.80 37 Rb 38 Sr 39 Y 40 Zr 41 Nb 42 Mo 43 Tc 44 Ru 45 Rh 46 Pd 47 Ag 48 Cd 49 In 50 Sn 51 Sb 52 Te 53 I 54 Xe 85.47 87.62 88.91 91.22 92.91 95.95 (98) 101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.3 55 Cs 56 Ba 57 La 72 Hf 73 Ta 74 W 75 Re 76 Os 77 Ir 78 Pt 79 Au 80 Hg 81 Tl 82 Pb 83 Bi 84 Po 85 At 86 Rn 132.9 137.3 138.9 178.5 180.9 183.8 186.2 190.2 192.2 195.1 197.0 200.6 204.4 207.2 209.0 (209) (210) (222) 87 Fr 88 Ra 89 Ac 104 Rf 105 Db 106 Sg 107 Bh 108 Hs 109 Mt 110 Ds 111 Rg 112 Cn 113 Nh 114 Fl 115 Mc 116 Lv 117 Ts 118 Og (223) (226) (227) (261) (262) (263) (262) (265) (266) (281) (272) (285) (286) (289) (289) (293) (294) 58 Ce 59 Pr 60 Nd 61 Pm 62 Sm 63 Eu 64 Gd 65 Tb 66 Dy 67 Ho 68 Er 69 Tm 70 Yb 71 Lu 140.1 140.9 144.2 (145) 150.4 152.0 157.3 158.9 162.5 164.9 167.3 168.9 173.0 175.0 90 Th 91 Pa 92 U 93 Np 94 Pu 95 Am 96 Cm 97 Bk 98 Cf 99 Es 100 Fm 101 Md 102 No 103 Lr 232.0 231.0 238.0 (237) (244) (243) (247) (247) (251) (252) (257) (258) (259) (262) (294) DIRECTIONS  When you have selected your answer to each question, blacken the corresponding space on the answer sheet using a soft, #2 pencil. Make a heavy, full mark, but no stray marks. If you decide to change an answer, erase the unwanted mark very carefully.  There is only one correct answer to each question. Any questions for which more than one response has been blackened will not be counted.  Your score is based solely on the number of questions you answer correctly. It is to your advantage to answer every question. 1. At 120 °C and 1 atm pressure, 1.00 L of methane, CH 4, reacts completely with excess oxygen to form carbon dioxide and water. What volumes of the two products are produced at this pressure and temperature? (A) 1.00 L CO2 and 2.00 L H2O (B) 1.00 L CO2 and 4.00 L H2O 6. The concentration of an aqueous solution of a nonvolatile, monoprotic acid is measured first by freezing point depression and then by boiling point elevation. The solution is found to be 0.93 m by freezing point depression and to be 0.82 m by boiling point elevation. Which is the best explanation for this discrepancy? (A) Ionization of the acid is markedly exothermic. (C) 2.00 L CO2 and 2.00 L H2O (B) The solute associates partially into dimers at lower temperatures. (D) 2.00 L CO2 and 4.00 L H2O 2. Polypropylene is made by polymerizing propene, C 3H 6 (M = 42.1). How many molecules of propene must be polymerized to make 3.50 g polypropylene? (A) 1.43 × 10 22 (B) 5.01 × 10 22 (C) 6.02 × 10 23 (D) 2.11 × 10 24 3. An organic compound contains only carbon, hydrogen, nitrogen, and oxygen. It is 61.71% C, 4.03% H, and 16.00% N by mass. What is its empirical formula? (A) C5H4NO (B) C9H7N2O2 (C) C10H8N2O (D) C11H8NO2 4. 100.0 mL of 0.500 M CaBr 2 and 50.0 mL of 1.00 M NaBr are mixed. What is the concentration of bromide ion in the resulting solution? (C) The volume of the solution is greater at higher temperatures. (D) The boiling point elevation constant for water is smaller than its freezing point depression constant. 7. When substances are separated by fractional distillation, which property is least typical of the substance that distills first? (A) Highest molar mass (B) Weakest intermolecular forces (C) Greatest vapor pressure (D) Lowest boiling point 8. Which element is most abundant (by mass) in the Earth’s crust? (A) 0.500 M (B) 0.667 M (A) Carbon (B) Oxygen (C) 0.750 M (D) 1.00 M (C) Magnesium (D) Silicon 5. 1.00 g of hydrated potassium carbonate, K 2CO3•nH 2O, is heated to 250 °C to give 0.836 g anhydrous K2CO3. What is the value of n? (A) 0.16 (B) 1.0 (C) 1.5 (D) 2.0 9. Which gas has the highest molar solubility in water at 25 °C and 1 atm? (A) CO2 (B) NH3 (C) O2 10. Which compound is colorless? (A) NaMnO4 (B) CrAsO4 (C) RbIO4 (D) BaCrO4 (D) H2S 11. Chlorine gas is bubbled into a colorless aqueous solution of sodium iodide. Which is the best description of what takes place? (A) A precipitate of white NaCl forms. 15. What is the principal intermolecular force that must be overcome when n-hexane (C6H14, bp = 69 °C) is vaporized? (A) Hydrogen bonding (B) A precipitate of metallic Na forms. (B) Covalent bonding between carbon atoms (C) The solution turns pale green as the chlorine dissolves. (C) Dipole-dipole forces (D) The solution turns yellow-brown as iodide reacts with the chlorine. 12. A student determines the number of moles of water in a hydrated metal oxide by weighing a clean, dry crucible and lid while the crucible is empty, then reweighing the crucible and lid with a sample of the hydrate, heating the crucible and lid with a Bunsen burner and then reweighing the crucible and lid with the sample after cooling to room temperature. Which error will result in too high a value for the amount of water of hydration? (A) The heating is conducted only once instead of the three times recommended by the procedure. (B) The lid is left off the crucible when it is weighed with the hydrated oxide. (C) The metal oxide reacts partially with oxygen in the air, forming a compound in a higher oxidation state. (D) Some of the heated oxide is spilled from the crucible before it can be weighed. 13. The diagram is a microscopic view of a snapshot of a substance at equilibrium, with the circles representing molecules and the arrows the molecules’ velocities. What state of matter is depicted? (D) London dispersion forces 16. The melting point of water decreases with increasing pressure. Which is the best explanation for this observation? (A) Liquid water is denser than solid water at 0 °C. (B) Melting of ice is endothermic at 0 °C. (C) The vapor pressure of liquid water is lower than the vapor pressure of solid water at 0 °C. (D) Solid and liquid water cannot coexist at equilibrium at 0 °C at pressures different from 1 atm. 17. A cylinder containing a mixture of CO and CO2 has a pressure of 2.00 atm at 93 °C (366 K). The cylinder is then cooled to –90 °C (183 K), where CO is still a gas but CO2 is a solid with a vapor pressure of 0.25 atm. The pressure in the cylinder at this temperature is 0.90 atm. What is the mole fraction of CO2 in the cylinder? (A) 0.10 (B) 0.28 (C) 0.35 (D) It cannot be determined from the information given. 18. A mineral containing only copper and oxygen adopts the cubic unit cell shown below. What is the formula of this mineral? (A) Solid (B) Liquid (C) Gas (D) Plasma 14. The vapor density of which fluorocarbon is 6.17 g L–1 at 23 °C and 1.00 atm? (A) C3F6 (B) C3F8 (C) C4F6 (D) C4F8 (A) Cu2O (B) CuO (C) Cu3O2 (D) Cu4O9 19. Burning 48.0 g of graphite in excess oxygen under standard conditions releases 1574.0 kJ of heat. What is ∆H°f of CO2(g)? (A) 1574 kJ mol -1 (B) –1574 kJ mol -1 (C) –394 kJ mol -1 (D) 32.8 kJ mol -1 20. Two metal samples, labeled A and B, absorb the same amount of heat. Sample A has a mass of 10.0 g, and its temperature increases by 38 °C. Sample B has a mass of 20.0 g, and its temperature increases by 23 °C. Which sample has the greater specific heat capacity? (A) Sample A (B) Sample B (C) Both samples have the same specific heat capacity. (D) It is impossible to determine from the information given. 21. A reaction has ∆S° > 0 and ∆H° > 0. Which statement about this reaction must be correct? (A) If carried out in a well-insulated flask, the temperature of the reaction mixture will decrease. (B) It will occur spontaneously at 298 K and 1 atm pressure. (C) As the temperature is raised, Keq for this reaction decreases. (D) The reaction has more moles of products than it has moles of reactants. 22. What is the boiling point of water in a pressure cooker with a pressure of 2.00 atm? (The enthalpy of vaporization of water is 40.7 kJ mol –1.) (A) 101 °C (B) 121 °C (C) 141 °C (D) 200 °C 23. The gas-phase bromination of propene has a standard enthalpy of reaction of –122.5 kJ mol-1. C3H6(g) + Br2(g) → C3H6Br2(g) ∆H° = –122.5 kJ mol-1 Substance C3H6(g) Br2(g) ∆H°f, kJ mol-1 20.4 30.9 What is the standard enthalpy of formation of C3H6Br2(g)? (A) –71.2 kJ mol -1 (B) –102.1 kJ mol -1 (C) –142.9 kJ mol-1 (D) –173.8 kJ mol -1 24. Titanium has a normal melting point of 1668 °C and a molar enthalpy of fusion of 14.15 kJ mol-1. The standard molar entropy of liquid titanium is 97.53 J mol-1 K-1 at 1668 °C. What is the standard molar entropy of solid titanium at this temperature? (A) 89.05 J mol -1 K-1 (B) 90.24 J mol -1 K -1 (C) 97.52 J mol -1 K-1 (D) 104.82 J mol -1 K-1 25. In the reaction Cl2(g) + 3 F2(g) → 2 ClF3(g) the rate of disappearance of F2(g) is 1.0 M s-1. What is the rate of appearance of ClF3(g)? (A) 0.33 M s -1 (B) 0.67 M s -1 (C) 1.0 M s -1 (D) 1.5 M s -1 26. When the rate of the reversible reaction A + B  C is studied under a certain set of conditions, it is found that the rate of the forward reaction is kf[A]. What can be concluded about the rate law for the reverse reaction under these conditions? (A) Rate = kr[C] (B) Rate = kr[C] [B] (C) The rate law of the reverse reaction cannot be determined from the information given. (D) An error must have been made, since if the reaction is reversible, the forward rate law must be Rate = kf[A][B]. 27. A substance A decomposes irreversibly to form B. A plot of ln([A]) as a function of time from the beginning of the reaction until A is 97% consumed is a straight line with a negative slope. What is the reaction order in A? (A) Zero order (B) First order (C) Second order (D) Third order 28. Which statement about catalysis is correct? (A) If a catalyst increases the forward rate of a reaction by a factor of two, it must increase the rate of the reverse reaction by a factor of two. (B) If a catalyst increases the rate of formation of a product by a factor of two, it must increase the rate of formation of the mirror image of the product by a factor of two. (C) A catalyst must be in the same phase as the reactants and products of the reaction. (D) A catalyzed reaction must proceed by the same mechanism as the uncatalyzed reaction, but with a lower activation energy. 226 29. The isotope Ra has a half-life for radioactive decay of 1600 y. How long will it take the amount of 226Ra in a 226 sample of RaCl2 to decrease by 25%? (A) 660 y (B) 800 y (C) 1200 y (D) 1600 y 30. The oxidation of sulfite ion by triiodide ion is proposed to take place by the following mechanism: I 3–  I2 + I – SO3 + I2 → ISO3– + I– – ISO3 + H2O → HSO4– + I– + H+ fast, unfavorable slow fast 2- What rate law is predicted by this mechanism? (A) Rate = k[I3–] (D) Rate = H2 (g) + I2(g)  2 HI(g) 2− – ][SO 끫殰[I 3 3 ] – [I ] Keq = ??? Into a 1.00 L vessel, 1.00 mol H2(g) and 1.00 mol I2(g) are placed at a high temperature. When the reaction mixture stops changing, it is found that 79.0% of the H2(g) has reacted. What is the equilibrium constant for this reaction at this temperature? (A) 14.2 (B) Rate = k[I3–][SO32- ] (C) Rate = 35. Consider the reaction: (B) 17.9 (C) 35.8 (D) 56.6 36. A triprotic acid is titrated with sodium hydroxide to give the titration curve below. What are the pKa values of the three acidic hydrogens? 2− – ][SO 끫殰[I 3 3 ] [H + ] 31. What is the correct equilibrium expression for the given reaction? Cu(OH)2(s) + 4 NH3(aq)  Cu(NH3)4 2+(aq) + 2 OH –(aq) (A) Keq = (B) Keq = (C) Keq = (D) Keq = – [Cu(NH3 )2+ 4 ][OH ] [NH3 ] 2+ [Cu(NH3 )4 ][2 OH– ] [4 NH3 ] – 2 [Cu(NH3 )2+ 4 ][OH ] [NH3 ] 4 – 2 [Cu(NH3 )2+ 4 ][2 OH ] [4 NH3 ] 4 -4 32. 0.100 mol of HF (K a = 6.6 × 10 ) is added to water to make 1.00 L of solution. Which statement is correct at equilibrium? (A) [H3 O +] = [HF] (C) [HF] > [F– ] (B) [H3 O +] = 0.100 M (D) [H3O + ] > [HF] 33. What is the solubility of silver oxalate, Ag 2C 2O 4 (Ksp = 3.5 × 10 -11), in a 0.050 M sodium oxalate solution? (A) 2.6, 5.0, 8.2 (B) 3.8, 6.6, 10.6 (C) 3.8, 8.2, 12.7 (D) 5.0, 8.2, 12.7 37. What is the average oxidation state of tin in the mineral abhurite, Sn21Cl16(OH)14O6? (A) +1.71 (B) +2.00 (C) +2.76 (D) +3.43 38. If used in an electrochemical cell, which of the following half-reactions would require an inert electrode? (A) 1.4 × 10-8 M (B) 5.9 × 10 -6 M I. 2 Cl–(aq) ⇄ Cl2(g) + 2 e– II. Fe(CN)63- (aq) + e – ⇄ Fe(CN)64-(aq) (C) 1.3 × 10-5 M (D) 2.1 × 10 -4 M (A) I only (B) II only (C) Both I and II (D) Neither I nor II 34. A pure sample of a monoprotic acid is dissolved in water. The sample is titrated with sodium hydroxide solution. At the point where 20.0 mL of the NaOH solution has been added, the pH is 4.15. The phenolphthalein endpoint of the titration is observed when 50.0 mL of NaOH have been added. What is the pKa of the acid? (A) 4.15 (B) 4.33 (C) 4.55 (D) 5.19 39. Chromium is electroplated industrially by the electrolysis of solutions of K2Cr2O7. How much time would be required to deposit 1.00 kg of Cr using a current of 200.0 A? (A) 2.58 h (B) 7.74 h (C) 15.5 h (D) 31.0 h 40. What is the standard reduction potential of Cr3+(aq) to form metallic chromium? Half-reaction Cr3+(aq) + e– → Cr2+(aq) 2+ Cr (aq) + 2e– → Cr(s) Cr3+(aq) + 3e– → Cr(s) E°, V –0.41 –0.91 ??? (A) –0.74 V (B) –1.32 V (C) –1.73 V (D) –2.23 V 41. What is the equilibrium constant at 298 K for the disproportionation of uranium(V) in acidic solution as shown below? 2 UO2 +(aq) + 4 H +(aq) ⇄ UO22+(aq) + U 4+(aq) + 2 H2O(l) Half-reaction UO22+(aq) + e– → UO2+(aq) – + + UO2 (aq) + 4 H (aq) + e → U4+(aq) + 2 H2O(l) Keq = ??? E°, V +0.16 +0.27 (A) 1.9 × 10-4 (B) 73 (C) 5300 (D) 1.9 × 10 7 (B) +1.42 V (C) +1.04 V (D) –0.85 V (A) 3, 2, 2, 1/2 (B) 3, 1, –1, 1/2 (C) 2, 1, 0, 1/2 (D) 3, 2, 0, – 1/2 44. Which period 3 element has these successive ionization energies (in kJ mol-1)? (A) Al (B) Si IE3 3231.6 IE7 23780 (C) P (C) 1s 2 2s22p 63s2 3p64s 24d 5 (D) 1s 2 2s22p 63s2 3p63d 7 46. A fluorescent dye absorbs a photon of light of 485 nm and emits a photon of light at 540 nm. How much energy is lost as heat? (A) 4.17 × 10 -20 J (B) 3.68 × 10 -19 J (C) 4.10 × 10-19 J (D) 3.06 × 10 -18 J 47. Which gas-phase atom releases the most energy when an electron is added to it? (A) Na (B) Cl (C) K (D) Br 48. The isotope 44Ti undergoes electron capture. Which daughter isotope is produced? 40K (B) 40Ca (C) 44Sc (D) 44V (A) CO (B) CO2 (C) CH2O (D) CH3OH 50. Which molecule has a nonzero dipole moment? (A) O2 (B) O3 (C) S8 (D) SO3 51. Which statement best describes the structure and bonding in nitromethane, H3C–NO2? 43. Which is a possible set of quantum numbers n, l, m l, m s for a valence electron of sulfur (S)? IE2 1577.1 IE6 19805 (B) 1s 2 2s22p 63s2 3p64s 23d 5 49. Which species has the strongest carbon-oxygen bond? E° = +1.51 V (A) +1.50 V IE1 786.5 IE5 16091 (A) 1s 2 2s22p 63s2 3p64s 23d 7 (A) 42. The standard reduction potential E° for the reduction of permanganate in acidic solution is +1.51 V. What is the reduction potential for this half-reaction at pH = 5.00? + – – MnO 4 (aq) + 8 H (aq) + 5 e → Mn 2+(aq) + 4 H2O(l) 45. Which is the ground-state electron configuration of gasphase Co2+ ? IE4 4355.5 (D) S (A) Each of the three bonds to nitrogen is a different length. (B) The molecule is Lewis acidic because the nitrogen does not obey the octet rule. (C) The molecule is Lewis basic because the nitrogen has a lone pair. (D) The nitrogen atom has a trigonal planar geometry. 52. Which statements about the superoxide ion, O2 –, are correct? I. It has a bond order of 2.0. II. It has exactly one unpaired electron. (A) I only (B) II only (C) Both I and II (D) Neither I nor II 53. Which structure best depicts the three-dimensional shape of the P4 molecule? 59. Which conformation of cyclohexane (C6 H12 ) is most stable? (A) (B) (A) (B) (C) (D) (C) (D) 60. Which element is NOT present in DNA? 54. How many isomers are there of the octahedral coordination complex Co(NH3)3Cl3? (A) 1 (B) 2 (C) 3 (A) H (B) N (C) P (D) 4 55. Which is NOT a valid representation of 3-methyl-1butene? (A) (CH3)2CHCH=CH2 (B) CH2CHCH(CH3)2 (C) (D) 56. What is the relationship between the two compounds shown? (A) Identical (B) Structural isomers (C) Geometric isomers (D) Mirror image isomers 57. An amine with the formula C 4H 11N will have which of these properties? (A) Unpleasant odor (B) Boiling point greater than 100 °C (C) Absorption of light with λ > 45 0 nm (D) Water solubility less than 10 g/L 58. Which combination of reactants and catalyst will produce methyl propanoate, CH3CH2COOCH3, upon heating? (A) CH3CH2OH and CH3COOH with catalytic NaOH (B) CH3CH2OH and CH3COOH with catalytic H2SO4 (C) CH3OH and CH3CH2COOH with catalytic NaOH (D) CH3OH and CH3CH2COOH with catalytic H2SO4 END OF TEST (D) S KEY Number 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. Answer A B B D C A A B B C D D B A D A C A C A A B A B B B B A A C Number 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. Answer C C C B D A B C C A B C B B D A B C A B D B D B C C A D B D