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__________________________ Name: Date: _____________ Version B Part 1. Calculation/Short Answer Problems. All work for the following problems must be done in THIS TEST BOOKLET to receive credit. Calculations done with no work will receive zero points. You work must be clear and logical AND SHOWN to receive maximum (or any) credit. A. For the reaction: SO2Cl2(g) SO2(g) + Cl2(g) at 375 K, the equilibrium constant KP is 2.40. In an experiment, a sample of pure SO2Cl2 gas is placed in a sealed reaction vessel. The initial pressure of SO2Cl2 is 5.00 atm. The reaction is allowed to come to equilibrium. What are the final equilibrium pressures of all the gases? You must set up and use an equilibrium table. (Hint: the quadratic equation is needed.) (25 points) initial change equilibrium SO2Cl2(g) 5.00 x 5.00 x SO2(g) + 0 +x x Cl2(g) 0 +x x K= PSO2 PCl2 PSO2Cl 2 2.40 = x2 5.00 x = 2.40 x 2 + 2.40x 12.00 = 0 x= ( 2.40 ) 2 4 (1) ( 12.00 ) 2(1) = 2.40 7.33 2 x = 2.47 ( or 4.87 ) PSO2Cl2 = 5.00 x = 5.00 2.47 = 2.53 atm PSO2 = x = 2.47 atm PCl2 = x = 2.47 atm Page 1 Version B Name_______________________________ Student ID_________________________ B. A chemist makes up a solution of ascorbic acid (vitamin C, with formula C6H8O6) in water. C6H8O6(aq) + H2O(l) C6H7O6(aq) + H3O+(aq) Using a pH meter, she measures the pH of the solution at 2.5. a) What is the H3O+ ion concentration? (4 points) pH = log H 3O + H 3O + = 10 pH = 102.5 = 3.16 103 M b) Knowing the Ka for ascorbic acid is 8.0 105, calculate the equilibrium concentration of C6H8O6 and of the ion C6H7O6. (16 points) [H3O+] = 3.16 10-3 ; [C6H8O6]init = y C6H8O6(aq) + H2O(l) C6H7O6(aq) + H3O+(aq) initial y 0 0 change (3.16 10-3) +3.16 10-3 +3.16 10-3 equilibrium y (3.16 10-3) 3.16 10-3 3.16 10-3 C 6 H 7 O 6 H 3O + ( 3.16 103 ) = 8.0 105 Ka = = y 3.16 103 [ C6 H8O6 ] 2 ( 3.16 10 ) (8.0 10 ) y + ( 2.54 10 ) = 0 ( 3.16 10 ) + ( 2.54 10 ) = 1.03 10 = 1.28 10 y= 8.0 10 (8.0 10 ) 3 2 3 7 3 2 3 5 3 3 -1 [C6 H8O6 ]init = 1.28 10-1 [C6 H8O6 ] = y ( 3.16 103 ) = (1.28 10-1 ) [ C6 H8 O 6 ] = 1.25 101 M C6 H 7 O6 = H 3O + = 3.16 103 M ( 3.16 103 ) c) Calculate the percentage deprotonization of ascorbic acid. (5 points) 3 H 3O + 100% = 3.16 10 100% = 2.47% 1.28 10-1 [ C6 H8O6 ]init Page 2 Version B Name_______________________________ Student ID_________________________ C. A 0.10 mole sample of potassium cyanide salt, KCN, is dissolved in water and diluted to 1.00 liter total volume. a) Write the balanced equilibrium equation for cyanide ion in water. (5 points) CN(aq) + H2O(l) HCN(aq) + OH(aq) b) Knowing that the Ka of hydrocyanic acid, HCN, is 4.91010, calculate the value of Kb for CN. (6 points) K w = K K a b = 1.0 10 14 Kb = Kw 1.0 1014 = = 2.0 105 10 Ka 4.9 10 c) What is the pH of the solution described by the equilibrium reaction in part a. (Hint: the percent protonated is less than 5%) (14 points) CN(aq) + H2O(l) HCN(aq) + OH(aq) initial 0.10 0 0 change x +x +x equilibrium 0.10 x x x 2 [ HCN ] OH x = Kb = = 2.0 105 0.10 x CN x2 Kb = 2.0 105 0.10 2 x = 2.0 106 ; x = 1.4 103 + pOH = log (1.4 103 ) = 2.85 pH = 14 2.85 = 11.15 x = OH = 1.4 103 K w = H 3O OH Kw 1.0 1014 H 3O + = = = 7.11012 1.4 103 OH pH = log ( 7.1 1012 ) = 11.15 Part 2.Calculation/Short Answer problems (5 points each). Select the best answer from the given choices and MARK IT ON YOUR SCANTRON. 1. Calculate G at 298 K for the reaction C2H5OH(l) C2H5OH(g, 0.0400 bar) given G = 6.2 kJ at 298 K. A) 14 kJ B) 2.7 kJ C) 14 kJ D) 1.8 kJ E) 1.8 kJ Page 3 Version B Name_______________________________ Student ID_________________________ 2. Write the equilibrium constant for 2NaBr(aq) + Pb(ClO4)2(aq) b PbBr2(s) + 2NaClO4(aq). A) K = [Pb2+][Br]2 B) K = 1/([Pb2+][Br]2) CORRECT ANSWER C) K = [NaClO4]2/([NaBr]2[Pb(ClO4)2] ACCEPTED ANSWER D) K = [PbBr2]/([Pb2+][Br]2) E) K = 1/([Pb(ClO4)2][NaBr]2) 3. Calculate the value of K at 700 K for the reaction H2(g) + I2(g) b 2HI(g) given that Kc = 54 at the same temperature. A) 3100 B) 2.2 C) 54 D) 9.3 E) 1300 4. Which of the following produces the strongest conjugate base? A) HF (pKa = 3.45) B) HClO (pKa = 7.53) C) HCOOH (pKa = 3.75) D) CH3COOH (pKa = 4.75) E) HIO (pKa = 10.64) 5. Calculate the vapor pressure at 25C of a mixture of benzene and toluene in which the mole fraction of benzene is 0.650. The vapor pressure at 25C of benzene is 94.6 Torr and that of toluene is 29.1 Torr. A) 84.4 Torr B) 124 Torr C) 51.3 Torr D) 71.7 Torr E) 61.5 Torr 6. Which of the following equilibrium reactions is not affected by changes in pressure? A) 2BrCl(g) b Br2(g) + Cl2(g) B) H2(g) + Br2(l) b 2HBr(g) C) 2H2O2(l) b 2H2O(l) + O2(g) D) H2(g) + I2(s) b 2HI(g) E) 2CO2(g) b 2CO(g) + O2(g) Page 4 Version B Name_______________________________ Student ID_________________________ 7. The phase diagram for a pure substance is given below. The solid sublimes A) B) C) D) E) at 400 K and 200 atm. at 200 K and 100 atm. at 300 K and 100 atm. at 300 K and 75 atm. if warmed at any pressure below 50 atm. 8. What is the molality of CrCl3 in a solution prepared by dissolving 75.2 g chromium(III) chloride hexahydrate in 250.0 g of water. A) 0.282 m B) 1.13 m C) 7.60 m D) 5.64 m E) 1.90 m 9. Given: SO2(g) b O2(g) + S(s) SO3(g) b 1/2O2(g) + SO2(g) Calculate Kc for the reaction 2S(s) + 3O2(g) b 2SO3(g) A) B) C) D) E) 1.6 10103 1.6 1080 1.0 10130 1.6 1040 1.0 1065 Kc = 2.5 1053 Kc = 4.0 1013 Page 5 ... View Full Document

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