Hour Exam III Page 1 1. My answers for this Chemistry 102 exam should be graded with the answer sheet associated with: a) Form A b) Form B c) Form C d) Form D e) Form E Consider the titration of 30.0 ml of 0.30 M HCN by 0.10 M KOH for the next five questions. K a for HCN = 6.2 10 10. 2. Calculate the ph when 0.0 ml of KOH has been added. a) 9.73 b) 4.27 c) 3.61 d) 4.87 e) 5.29 3. Calculate the ph after 10.0 ml of KOH has been added. a) 8.30 b) 10.11 c) 9.09 d) 4.79 e) 9.21 4. Calculate the ph after 45.0 ml of KOH has been added. a) 8.30 b) 10.11 c) 9.09 d) 4.79 e) 9.21 5. Calculate the ph after 120.0 ml of KOH has been added. a) 12.00 b) 12.30 c) 12.50 d) 12.70 e) 13.00 6. Calculate the ph at the equivalence point in this titration. a) 11.04 b) 10.33 c) 12.08 d) 8.83 e) 7.00
Hour Exam III Page 2 7. H 2 CO 3 and H 2 SO 4 are both diprotic acids. For H 2 CO 3, K a 1 = 4.3 10 7 and K a 2 = 5.6 10 11. For H 2 SO 4, K a 1 >> 1 and K a 2 = 1.2 10 2. Which of the following statements concerning a 0.10 M H 2 CO 3 solution and a 0.10 M H 2 SO 4 solution is true? a) The [H + ] in the 0.10 M H 2 CO 3 solution will be greater than the [H + ] in the 0.10 M H 2 SO 4 solution. b) The [H + ] in the 0.10 M H 2 SO 4 solution will be less than 0.10 M ([H + ] < 0.10 M). c) At equilibrium, the [SO 2 4 ] will be greater than the [CO 2 3 ] ([SO 2 4 ] > [CO 2 3 ]). d) The ph of the 0.10 M H 2 CO 3 solution will be less than 1.0 (ph < 1.0). e) At equilibrium, [H 2 SO 4 ] > [HSO 4 ] in the 0.10 M H 2 SO 4 solution. 8 Consider a 0.10 M solution of a weak acid HA. If ph = 3.00 for this solution, determine the equilibrium concentration of the conjugate base ([A ] =?). a) 1.0 10 4 M b) 1.0 10 3 M c) 1.0 10 2 M d) 1.0 M e) 1.0 10 5 M 9. Which of the following statements is false? a) The ph of a 0.10 M solution of HClO 4 solution is 1.00. b) The ph of a 10. M solution of KOH is 15.00. c) The ph of a 1.0 10 12 M solution of HCl is 7.00. d) The ph of a 0.20 M solution of Ba(OH) 2 is 13.30. e) The ph of a 10. M solution of HCl is 1.00. 10. What is the ph of a 0.015 M solution of aniline, C 6 H 5 NH 2? K b for C 6 H 5 NH 2 is 3.8 10 10. a) 11.24 b) 7.00 c) 8.38 d) 2.76 e) 5.62 11. A weak acid, HA, is dissolved in water and no other substance is added. Which of these statements (to a close approximation) is true? a) [H 3 O + ] = [OH ] b) [HA] = [A ] c) [A ] = [OH ] d) [HA] = [OH ] e) [H 3 O + ] = [A ]
Hour Exam III Page 3 12. A sample of S 8 (g) is placed in an otherwise empty rigid container at 1325 K at an initial concentration of 1.000 M. The S 8 (g) decomposes to S 2 (g) by the reaction: S 8 (g) 4 S 2 (g) At equilibrium, the concentration of S 8 is 0.250 M. Calculate the value of K for this reaction at 1325 K. a. 324 b. 12.0 c. 1.27 d. 0.750 e. 0.100 13. Four different bases all with the same initial concentration and volume are each titrated with 0.1 M HNO 3. Which base titration has the highest ph at the halfway point to equivalence? a) CH 3 NH 2 (K b = 4.4 10 4 ) b) NH 3 (K b = 1.8 10 5 ) c) C 5 H 5 N (K b = 1.7 10 9 ) d) C 6 H 5 NH 2 (K b = 3.8 10 10 ) 14. The K a value for acetic acid (HC 2 H 3 O 2 ) is 1.8 10 5 and the K b value for ammonia (NH 3 ) is 1.8 10 5. Which of the following statements is true? a) A 1.0 M solution of NaC 2 H 3 O 2 would have a lower ph than a 1.0 M solution of NH 4 Cl. b) C 2 H 3 O 2 is a stronger base than NH 3. c) A 1.0 M solution of HC 2 H 3 O 2 would have a higher ph than a 1.0 M solution of NH 4 Cl. d) NH 4 + is a stronger acid than HC 2 H 3 O 2. e) A 1.0 M solution of NH 3 would have a higher ph than a 1.0 M solution of NaC 2 H 3 O 2. 15. Phosphate buffers are important in regulating the ph of intracellular fluids at ph = 7.10. The equilibrium reaction utilized in this buffer system is: H 2 PO 4 (aq) + H 2 O HPO 2 4 (aq) + H 3 O + (aq) K a = 6.2 10 8 At ph = 7.10, calculate the concentration ratio of HPO 2 4 to H 2 PO 4 in intracellular fluids 2 [HPO ] 4 =?. [H2PO4 ] a) 0.78 b) 0.11 c) 1.0 d) 9.1 e) 1.3
Hour Exam III Page 4 16. Calculate the [H 3 O + ] in a solution containing 0.50 M HNO 2 and 0.10 M NaNO 2. K a for HNO 2 = 4.0 10 4 a) 2.0 10 3 M b) 8.0 10 4 M c) 4.0 10 4 M d) 8.0 10 5 M e) 1.0 10 5 M 17. Consider 1.0 L of a solution containing 0.50 M HNO 2 and 0.10 M NaNO 2. How many moles of KOH must be added so that the resulting solution has ph = pk a for HNO 2? K a for HNO 2 = 4.0 10 4. a) 0.50 mol b) 0.40 mol c) 0.25 mol d) 0.20 mol e) 0.10 mol 18. Consider the following reaction at 250 C: N 2 (g) + 3 H 2 (g) 2 NH 3 (g) K = 0.278 Initially 1.00 mol of N 2, 1.00 mol of H 2, and 1.00 mol of NH 3 are placed in a 2.00 L container. Which way will the reaction shift to reach equilibrium and which reagent will have the smallest concentration at equilibrium? a. Reaction will shift right; [NH 3 ] will be smallest. b. Reaction will shift left; [H 2 ] will be smallest. c. Reaction will shift left; [NH 3 ] will be smallest. d. Reaction will shift right; [N 2 ] will be smallest. e. Reaction will shift right; [H 2 ] will be smallest. 19. How many of the following statements (I-IV) about a 1.0 M solution of weak acid is/are true? I. As the K a value increases, the ph of the solution decreases. II. As the K a value increases, the percent dissociation of the acid increases. III. As the K a value increases, the K b value of the conjugate base decreases. IV. As the K a value increases, the [H 3 O + ] in solution increases. a) 0 (none; all are false) b) 1 c) 2 d) 3 e) 4 (All are true)
Hour Exam III Page 5 20. Arrange 0.10 M solutions of the following compounds by increasing ph: HC 2 H 3 O 2 HF NH 3 C 5 H 5 N (K a = 1.8x10-5 ) (K a = 7.2x10-4 ) (K b = 1.8x10-5 ) (K b = 1.7x10-9 ) a) HF < HC 2 H 3 O 2 < NH 3 < C 5 H 5 N b) HF < HC 2 H 3 O 2 < C 5 H 5 N < NH 3 c) HC 2 H 3 O 2 < HF < NH 3 < C 5 H 5 N d) HC 2 H 3 O 2 < HF < C 5 H 5 N < NH 3 e) NH 3 < C 5 H 5 N < HF < HC 2 H 3 O 2 21. Which of the following reactions is correctly identified with the appropriate equilibrium constant? a) C 5 H 5 N(aq) + H 2 O(l) C 5 H 5 NH + (aq) + OH (aq) K a reaction for C 5 H 5 N b) NH 3 (aq) + H 3 O + (aq) NH 4 + (aq) + H 2 O(l) K b reaction for NH 3 c) H 3 O + (aq) + OH (aq) H 2 O(l) + H 2 O(l) K b reaction for H 3 O + d) HC 3 H 5 O 2 (aq) + H 2 O(l) C 3 H 5 O 2 (aq) + H 3 O + (aq) K a reaction for HC 3 H 5 O 2 22. How many of the following 0.25 M solutions are acidic? NaCN KI NH 4 Cl Ca(NO 3 ) 2 Na 2 O a. 0 (none of these solutions are acidic) b. 1 c. 3 d. 4 e. 5 (all of these solutions are acidic)
Hour Exam III Page 6 Consider the titration of 50.0 ml of 0.20 M ascorbic acid (H 2 C 6 H 6 O 6 ) by 0.20 M NaOH for the next two questions. For H 2 C 6 H 6 O 6, K a = 7.9 10 5 and K 1 a = 1.6 10 12. 2 23. At what volume of NaOH added does the resulting solution have [HC 6 H 6 O 6 ] = [C 6 H 6 O 6 2 ]? a) 0.0 ml NaOH added b) 25.0 ml NaOH added c) 50.0 ml NaOH added d) 75.0 ml NaOH added e) 100.0 ml NaOH added 24. Calculate the ph after 50.0 ml of NaOH has been added. a) 4.10 b) 6.71 c) 7.95 d) 9.23 e) 11.80 25. The poh of a sample of baking soda dissolved in water is 5.74 at 25 o C. Calculate the [H 3 O + ] in this baking soda solution. a) 1.0 10 7 M b) 5.5 10 9 M c) 1.8 10 6 M d) 1.9 10 9 M e) 6.2 10 6 M Consider the titration of 200.0 ml of 0.10 M HNO 3 titrated by 0.10 M Ca(OH) 2 for the next two questions. 26. Determine the [H + ] of the resulting solution after 50.0 ml of 0.10 M Ca(OH) 2 has been added. a) 0.10 M b) 0.050 M c) 0.040 M d) 0.020 M e) 0.010 M 27. At what volume of Ca(OH) 2 added does the resulting solution have [H + ] = 1.0 10 7 M? a) 100.0 ml b) 125.0 ml c) 150.0 ml d) 175.0 ml e) 200.0 ml
Hour Exam III Page 7 28. A 0.10 M solution of the salt NaX has a ph of 11.50. Calculate the ph of a 1.0 M solution of HX. a) 3.00 b) 4.00 c) 5.00 d) 6.00 e) 11.00 29. A chemist wishes to prepare a buffer at ph = 3.5 and has the following acids available, along with the potassium salts of their conjugate bases. Which acid/salt combination is the best choice for preparing the buffer? a) HC 2 H 3 O 2 (K a = 1.8 10 5 )/KC 2 H 3 O 2 b) HOCl (K a = 3.5 10 8 )/KOCl c) HNO 2 (K a = 4.0 10 4 )/KNO 2 d) HCN (K a = 6.2 10 10 )/KCN e) HClO 2 (K a = 1.2 10 2 )/KClO 2 30. Consider a buffer solution containing NaF and HF. Which of the following statements is false? a) If [F ] = [HF], then the ph of the solution equals the pk a value for HF. b) If [F ] > [HF], then the ph of the solution is greater than the pk a value for HF. c) If [HF] > [F ], then the [H 3 O + ] of the solution is greater than the K a value for HF. d) If HCl is added to the buffer, then the [F ] of the resulting solution should increase. e) If NaOH is added to the buffer, then the ph of the resulting solution should increase. 31. Consider the following endothermic reaction at equilibrium: SiHCl 3 (g) + 3 H 2 O(l) SiH(OH) 3 (s) + 3 HCl(g) ΔH = +450 kj How many of the following changes will cause the amount of SiH(OH) 3 (s) to increase? I. Increase the volume of the reaction container. II. Increase the temperature. III. Add H 2 O(l). IV. Remove HCl(g). a. 0 (none) b. 1 c. 2 d. 3 e. 4 (all)