CHEMISTRY 102 Fall 2010 Hour Exam III. 1. My answers for this Chemistry 102 exam should be graded with the answer sheet associated with:

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1 1. My answers for this Chemistry 10 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 = Calculate the ph when 0.0 ml of KOH has been added. a) 9.73 b) 4.7 c) 3.61 d) 4.87 e) Calculate the ph after 10.0 ml of KOH has been added. a) 8.30 b) c) 9.09 d) 4.79 e) Calculate the ph after 45.0 ml of KOH has been added. a) 8.30 b) c) 9.09 d) 4.79 e) Calculate the ph after 10.0 ml of KOH has been added. a) 1.00 b) 1.30 c) 1.50 d) 1.70 e) Calculate the ph at the equivalence point in this titration. a) b) c) 1.08 d) 8.83 e) 7.00 Page 4

2 7. H CO 3 and H SO 4 are both diprotic acids. For H CO 3, K a = and K 1 a = For H SO 4, K a >> 1 and K 1 a = Which of the following statements concerning a 0.10 M H CO 3 solution and a 0.10 M H SO 4 solution is true? a) The [H + ] in the 0.10 M H CO 3 solution will be greater than the [H + ] in the 0.10 M H SO 4 solution. b) The [H + ] in the 0.10 M H SO 4 solution will be less than 0.10 M ([H + ] < 0.10 M). c) At equilibrium, the [SO 4 ] will be greater than the [CO 3 ] ([SO 4 ] > [CO 3 ]). d) The ph of the 0.10 M H CO 3 solution will be less than 1.0 (ph < 1.0). e) At equilibrium, [H SO 4 ] > [HSO 4 ] in the 0.10 M H 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) M b) M c) M d) 1.0 M e) M 9. Which of the following statements is false? a) The ph of a 0.10 M solution of HClO 4 solution is b) The ph of a 10. M solution of KOH is c) The ph of a M solution of HCl is d) The ph of a 0.0 M solution of Ba(OH) is e) The ph of a 10. M solution of HCl is What is the ph of a M solution of aniline, C 6 H 5 NH? K b for C 6 H 5 NH is a) 11.4 b) 7.00 c) 8.38 d).76 e) 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 + ] = [OH ] b) [HA] = [A ] c) [A ] = [OH ] d) [HA] = [OH ] e) [H + ] = [A ] Page 5

3 1. Five different bases, all with the same initial concentration and volume, are each titrated with 0.1 M HNO 3. Which base titration has the lowest ph at the equivalence point? a) CH 3 NH (K b = ) b) NH 3 (K b = ) c) C 5 H 5 N (K b = ) d) C 6 H 5 NH (K b = ) e) NaOH 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 (K b = ) b) NH 3 (K b = ) c) C 5 H 5 N (K b = ) d) C 6 H 5 NH (K b = ) 14. The K a value for acetic acid (HC H 3 O ) is and the K b value for ammonia (NH 3 ) is Which of the following statements is true? a) A 1.0 M solution of NaC H 3 O would have a lower ph than a 1.0 M solution of NH 4 Cl. b) C H 3 O is a stronger base than NH 3. c) A 1.0 M solution of HC H 3 O would have a higher ph than a 1.0 M solution of NH 4 Cl. d) NH 4 + is a stronger acid than HC H 3 O. e) A 1.0 M solution of NH 3 would have a higher ph than a 1.0 M solution of NaC H 3 O. 15. Phosphate buffers are important in regulating the ph of intracellular fluids at ph = The equilibrium reaction utilized in this buffer system is: H PO 4 (aq) HPO 4 (aq) + H + (aq) K a = At ph = 7.10, calculate the concentration ratio of HPO 4 to H PO 4 in intracellular fluids [HPO ] 4?. [HPO4 ] a) 0.78 b) 0.11 c) 1.0 d) 9.1 e) 1.3 Page 6

4 Page Calculate the [H + ] in a solution containing 0.50 M HNO and 0.10 M NaNO. K a for HNO = a) M b) M c) M d) M e) M 17. Consider 1.0 L of a solution containing 0.50 M HNO and 0.10 M NaNO. How many moles of KOH must be added so that the resulting solution has ph = pk a for HNO? K a for HNO = a) 0.50 mol b) 0.40 mol c) 0.5 mol d) 0.0 mol e) 0.10 mol 18. Which of the following indicates the most acidic solution? a) ph = 1. b) [H + ] = M c) [H + ] = 0.3 M d) poh = 5.9 e) [OH ] = 0.5 M 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 + ] in solution increases. a) 0 (none) b) 1 c) d) 3 e) 4 [All of the statements (I-IV) are true.] 0. Arrange 0.10 M solutions of the following compounds by increasing ph: HC H 3 O HF NH 3 C 5 H 5 N a) HF < HC H 3 O < NH 3 < C 5 H 5 N b) HF < HC H 3 O < C 5 H 5 N < NH 3 c) HC H 3 O < HF < NH 3 < C 5 H 5 N d) HC H 3 O < HF < C 5 H 5 N < NH 3 e) NH 3 < C 5 H 5 N < HF < HC H 3 O Page 7

5 1. Which of the following reactions is correctly identified with the appropriate equilibrium constant? a) C 5 H 5 N(aq) + H 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 O(l) K b reaction for NH 3 c) H 3 O + (aq) + OH (aq) H O(l) + H O(l) K b reaction for H 3 O + d) HC 3 H 5 O (aq) + H O(l) C 3 H 5 O (aq) + H 3 O + (aq) K a reaction for HC 3 H 5 O Consider the titration of 50.0 ml of 0.0 M ascorbic acid (H C 6 H 6 O 6 ) by 0.0 M NaOH for the next two questions. For H C 6 H 6 O 6, K a = and K 1 a = At what volume of NaOH added does the resulting solution have [HC 6 H 6 O 6 ] = [C 6 H 6 O 6 ]? a) 0.0 ml NaOH added b) 5.0 ml NaOH added c) 50.0 ml NaOH added d) 75.0 ml NaOH added e) ml NaOH added 3. Calculate the ph after 50.0 ml of NaOH has been added. a) 4.10 b) 6.71 c) 7.95 d) 9.3 e) The poh of a sample of baking soda dissolved in water is 5.74 at 5 o C. Calculate the [H + ] in this baking soda solution. a) M b) M c) M d) M e) M Page 8

6 Page 9 Consider the following four titrations (labeled I-IV) for the next two questions. I ml of 0.10 M HC H 3 O titrated by 0.10 M NaOH (K a for HC H 3 O = ) II ml of 0.10 M NaOH titrated by 0.10 M HCl III ml of 0.10 M HCl titrated by 0.10 M NaOH IV ml of 0.10 M NH 3 titrated by 0.10 M HCl (K b for NH 3 = ) 5. Rank the four titrations (I-IV) in order of increasing ph at the halfway point to equivalence (lowest to highest ph). a) III < I = IV < II b) III < I < IV < II c) II = IV < I = III d) I = III < II = IV e) III < II < I < IV 6. Rank the four titrations (I-IV) in order of increasing ph at the equivalence point (lowest ph to highest ph). a) I = II = III = IV b) I < II = III < IV c) I = III < II = IV d) II = IV < I = III e) IV < II = III < I Consider the titration of 00.0 ml of 0.10 M HNO 3 titrated by 0.10 M Ca(OH) for the next two questions. 7. Determine the [H + ] of the resulting solution after 50.0 ml of 0.10 M Ca(OH) has been added. a) 0.10 M b) M c) M d) 0.00 M e) M 8. At what volume of Ca(OH) added does the resulting solution have [H + ] = M? a) ml b) 15.0 ml c) ml d) ml e) 00.0 ml 9. A 0.10 M solution of the salt NaX has a ph of Calculate the ph of a 1.0 M solution of HX. a) 3.00 b) 4.00 c) 5.00 d) 6.00 e) Page 9

7 30. 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 H 3 O (K a = )/KC H 3 O b) HOCl (K a = )/KOCl c) HNO (K a = )/KNO d) HCN (K a = )/KCN e) HClO (K a = )/KClO 31. A gardener wants to increase the ph of his soil. Which of the following compounds could the gardener add to the soil (along with water) in order to increase the ph? a) NH 4 Cl b) CaO c) P O 5 d) SrBr e) HF 3. Consider separate 0.5 M solutions of the following three salts: KNO, (C H 5 ) 3 NHNO 3, (C H 5 ) 3 NHNO Rank the three salt solutions in order of increasing ph (most acidic to most basic). [K a for HNO = and K b for (C H 5 ) 3 N = ]. a) (C H 5 ) 3 NHNO < KNO < (C H 5 ) 3 NHNO 3 b) KNO < (C H 5 ) 3 NHNO 3 < (C H 5 ) 3 NHNO c) KNO < (C H 5 ) 3 NHNO < (C H 5 ) 3 NHNO 3 d) (C H 5 ) 3 NHNO 3 < KNO < (C H 5 ) 3 NHNO e) (C H 5 ) 3 NHNO 3 < (C H 5 ) 3 NHNO < KNO 33. 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 + ] 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. Page 30

8 Page 31 USEFUL CONSTANTS/EQUATIONS K w = K a K b Acid K a K w = [H + ][OH ] K w = (at 5 C) ph + poh = (at 5 C) pk a + pk b = (at 5 C) ph = log [H + ], [H + ] = 10 ph ph = pk a + log [base] [acid] poh = log [OH ], [OH ] = 10 poh HF HC H 3 O HC 3 H 5 O Base NH 3 C 5 H 5 N % dissociation = K b amount reacted initialconcentrat ion 100 pk a = log K a ; pk b = log K b ; K b = K a = 10 pk a M A V A = M B V B 10 pk b ph = ph = pk pk a a pk 1 a pk a 3 Solubility rules: 1. Most nitrate salts are soluble.. Most salts of alkali metals and ammonium cations are soluble. 3. Most chloride, bromide and iodide salts are soluble. Exceptions: salts containing Ag +, Pb + and Hg + ions are insoluble. 4. Most sulfate salts are soluble. Exceptions: sulfates containing Ca +, Ba +, Pb + and Hg + ions are insoluble. 5. Most hydroxide salts are insoluble. Exceptions: hydroxides containing alkali metals, Ba +, Sr + and Ca + ions are soluble. 6. Most sulfide, carbonate, chromate and phosphate salts are insoluble. Exceptions: salts of alkali metals and ammonium cations are soluble. Page 31