Homework answers / question archive / Chapter 16  Equilibria in Solutions of Weak Acids and Bases  Multiple Choice  1)The ionization constant, Ka, for macnic acid is 5

Chapter 16  Equilibria in Solutions of Weak Acids and Bases  Multiple Choice  1)The ionization constant, Ka, for macnic acid is 5

Chemistry

Share With

---

Chapter 16  Equilibria in Solutions of Weak Acids and Bases

 Multiple Choice

 1)The ionization constant, K_a, for macnic acid is 5.0 x 10^-5.  What is the pK_a of this acid?

 

                  

 

 

2. The ionization constant, K_a, for lactic acid is 1.38 × 10^-4.  What is the pK_a of this acid?

 

 

 

3. The ionization constant, K_a, for benzoic acid is 6.28 × 10^-5.  What is the pK_a of this acid?

 

                  

 

 

4. The ionization constant, K_b, for ammonia has a value of 1.76 × 10^-5.  What is the pK_b of this base?

 

 

 

5. Hydrazine is a base, and the value of its constant, K_b, is 1.70 × 10^-6.  What is the value of pK_b for this base?

 

    

 

6. Butanoic acid, HC₄H₇O₂, has a pK_a value = 4.82.  What is the value of the ionization constant for butanoic acid?  

 

 

 

7. Hypochlorous acid, HClO, has a pK_a value = 82.  What is the value of the ionization constant for hypochlorous acid?  

 

 

 

8. Formic acid, HCO₂H, has an ionization constant with the value:  K_a = 1.76 × 10^-4.  Calculate the value of pK_b for the conjugate base of formic acid.

 

 

9. Benzoic acid, HC₇H₅O₂, has an ionization constant with the value:  K_a = 6.4 × 10^-5.  Calculate the value of pK_b for the conjugate base of benzoic acid.

 

 

 

10. Toluic acid, HC₈H₇O₂, has an ionization constant with the value:  K_a = 1.23 × 10^-5.  Calculate the value of pK_b for the conjugate base of toluic acid.

 

 

11. A 0.100 M solution of an acid, HA, has a pH = 2.00.  What is the value of the ionization constant, K_a for this acid?

 

 

12. A 0.400 M solution of an acid, HQ, has a pH = 1.301.  What is the value of the ionization constant, K_a, for this acid?

 

 

 

13. A 0.200 M solution of an acid has a pH = 1.00.  What is the value of the ionization constant, K_a, for this acid?

 

 

 

14. A 0.300 M solution of an acid, HZ, has a pH = 1.301.  What is the value of the ionization constant, K_a, for this acid?

 

 

 

 

15. A 0.200 M solution of a weak base in water has a pH = 10.40.  Calculate the value of K_b for this base.

 

           

 

 

16. The ionization constant, K_a, for benzoic acid, HC₇H₅O₂, is 6.28 × 10^-5.  What is the pH of a 0.15 molar solution of this acid?

 

 

 

17. The ionization constant, K_a, for acetic acid, HC₂H₃O₂, is 1.76 × 10^-5.  What is the pH of a 0.0800 molar solution of this acid?

 

 

 

18. What is the pH of a 1.00 molar solution of nitrous acid?  The K_a for nitrous acid is 7.1 x 10^-4.

 

 

 

 

19. The ionization constant, K_b, for the weak base trimethylamine, (CH₃)₃N, is 7.4 × 10^-5.  What is the pH of a 0.040 molar solution of this base?

 

20. The ionization constant, K_a, for HCN(aq) is 6.2 × 10^-10.  What is the pH of a 0.10 molar solution of sodium cyanide, which contains the cyanide ion?  

 

 

21. The ionization constant, K_a, for HOI(aq), hypoiodous acid, is 2.3 ×  10^-11.  What is the pH of a

0.050 molar solution of sodium hypoiodite, which contains the hypoiodite ion?  

 

 

22. What is the pH of a 1.00 molar solution of NaCN(aq)?  The K_a for HCN = 6.2 ×  10^-10.

 

 

23. What is the pH of a 1.00 molar solution of NaNO₂(aq)?  The K_a for nitrous acid is 7.0 ×  10^-4.  

 

 

24. What is the pH of a 1.00 molar solution of NaNO₃(aq)?  Nitric acid is a strong acid.  

 

 

 

25. What is the pH of a 1.00 molar solution of KCl?  Hydrochloric acid is a strong acid.

 

 

26. What is the pH of a 1.00 molar solution of NH₄Cl(aq)?  The K_b for NH₃ is 1.8 × 10^-5.

 

 

27. What is the pH of a 1.00 molar solution of NaC₂H₃O₂(aq)?  The K_a for acetic acid is 1.8 × 10^-5.

 

 

 

28. What is the pH of a 1.00 molar solution of NaBr(aq)? Hydrobromic acid is a strong acid.

 

 

 

29. What is the pH of a 1.00 x 10^-9 molar solution of NaCl?

 

 

30. The ionization constant, K_a, for dichloroacetic acid, HC₂HO₂Cl₂ , is 5.0 × 10^-2.  What is the pH of a

0.15 molar solution of this acid?

 

 

 

31. Trihydroxybenzoic acid (THB) is a not-so-weak acid with an ionization constant:  K_a = 2.1 × 10^-2.  Calculate the [H⁺] and the pH of a 0.050 molar solution of THB.  (Simplified assumptions do not work in this case) 

 

      

 

 

32. Dichloroacetic acid HC₂HCl₂O₂, is a not-so-weak acid with an ionization constant:  K_a = 5.5 × 10^-2.  Calculate the [H⁺] and the pH of a 0.100 molar solution of this acid.  (Simplified assumptions do not work in this case) 

 

 

 

33. A buffer solution is prepared by taking 0.400 moles of acetic acid (pK_a = 4.76) and 0.250 moles of sodium acetate in sufficient water to make 1.400 liters of solution.  Calculate the pH of this solution.

 

  

 

 

34. A buffer solution is prepared by taking 0.250 moles of acetic acid (pK_a = 4.76) and 0.400 moles of sodium acetate in sufficient water to make 1.800 liters of solution.  Calculate the solution pH. 

 

 

35. A buffer solution is prepared by taking 0.400 moles of acetic acid (pK_a = 4.76) and 0.250 moles of calcium acetate in sufficient water to make 1.400 liters of solution.  Calculate the pH of this solution.

 

 

36. A buffer solution is prepared by taking 0.250 moles of acetic acid (pK_a = 4.76) and 0.400 moles of barium acetate in sufficient water to make 1.400 liters of solution.  Calculate the pH of this solution.

 

 

 

37. The pK_a of acetic acid, HC₂H₃O₂, is 4.76.  A buffer solution was made using an unspecified amount of acetic acid and 0.30 moles of NaC₂H₃O₂ in enough water to make 2.00 liters of solution.  Its pH was measured as 4.40.  How many moles of HC₂H₃O₂ were used?

 

 

 

38. The pK_a of acetic acid, HC₂H₃O₂, is 4.76.  A buffer solution was made using an unspecified amount of acetic acid and 0.30 moles of NaC₂H₃O₂ in enough water to make 1.50 liters of solution.  Its pH was measured as 4.50 on a meter.  How many moles of HC₂H₃O₂ were used?

 

 

 

 

39. The pK_a of acetic acid, HC₂H₃O₂, is 4.76.  A buffer solution was made using an unspecified amount of NaC₂H₃O₂ and 0.30 moles of acetic acid in enough water to make 2.00 liters of solution.  Its pH was measured as 4.60.  How many moles of NaC₂H₃O₂ were used?

 

 

 

40. The pK_a of acetic acid, HC₂H₃O₂, is 4.76.  A buffer solution was made using an unspecified amount of NaC₂H₃O₂ and 0.30 moles of acetic acid in enough water to make 1.50 liters of solution.  Its pH was measured as 4.55.  How many moles of NaC₂H₃O₂ were used?

 

                

 

 

41. For H₃PO₃, K_a1 = 1.0 × 10^-2 and K_a2 = 2.6 × 10^-7.  Calculate a value for the [H₂PO₃^-] in a 0.500 molar solution of H₃PO₃.

 

                                                

 

 

42. For H₃PO₃, which is actually a diprotic acid, K_a1 = 1.0 × 10^-2 and K_a2 = 2.6 x 10^-7.  Calculate a value for the [H⁺] in a 0.500 molar solution of H₃PO₃.

 

                          

 

 

43. For H₃PO₃, which is actually a diprotic acid, K_a1 = 1.0 × 10^-2 and K_a2 = 2.6 × 10^-7.  Calculate a value for the [HPO₃^2-] in a 0.500 molar solution of H₃PO₃.

 

                

 

 

44. Which is the most appropriate acid-base indicator to use in the titration of 0.100 molar HCl(aq) by

0.100 molar NaOH(aq)?  The NaOH(aq) is the titrant.

 

 

 

45. A mixture contains 0.060 moles of NaH₂PO₄ and 0.080 moles of K₂HPO₄.  It is titrated with 0.500 molar NaOH(aq) to neutralize it completely.  How many mL of the NaOH solution are required?

 

 

 

46. For H₃PO₄, a tripotic acid, K_a1 = 7.1 × 10^-3, K_a2 = 6.3 × 10^-8, and K_a3 = 4.5 × 10^-13.  What is the pH of a solution which contains 0.200 moles of K₂HPO₄ and 0.300 moles of Na₃PO₄ per liter of solution?

 

 

 

47. For H₃PO₄, a tripotic acid, K_a1 = 7.1 × 10^-3, K_a2 = 6.3 × 10^-8, and K_a3 = 4.5 × 10^-13.  What is the pH of a solution which contains 0.300 moles of Na₂HPO₄ and 0.200 moles of Na₃PO₄ per liter of solution?

 

 

 

48. For H₃PO₄, a tripotic acid, K_a1 = 7.1 × 10^-3, K_a2 = 6.3 × 10^-8, and K_a3 = 4.5 × 10^-13.  What is the pH of a solution which contains 0.400 moles of Na₂HPO₄ and 0.300 moles of K₃PO₄ per liter of solution?

 

 

 

49. For H₃PO₄, a tripotic acid, K_a1 = 7.1 × 10^-3, K_a2 = 6.3 × 10^-8, and K_a3 = 4.5 × 10^-13.  What is the pH of a solution which contains 0.300 moles of Na₂HPO₄ and 0.400 moles of KH₂PO₄ per liter of solution?

 

             

 

 

50. For H₃PO₄, a tripotic acid, K_a1 = 7.1 × 10^-3, K_a2 = 6.3 × 10^-8, and K_a3 = 4.5 × 10^-13.  What is the pH of a solution which contains 0.300 moles of Na₂HPO₄ and 0.200 moles of NaH₂PO₄ per liter of solution?

 

 

 

51. For H₃PO₄, a tripotic acid, K_a1 = 7.1 × 10^-3, K_a2 = 6.3 × 10^-8, and K_a3 = 4.5 × 10^-13.  What is the pH of a solution which contains 0.400 moles of K₂HPO₄ and 0.300 moles of NaH₂PO₄ per liter of solution?

 

 

 

53. Carbonic acid, H₂CO₃, is a diprotic acid with ionization constants:  K_a1 = 4.5 × 10^-7, K_a2 = 4.7 × 10^-11.  What is the value of the ratio, [H₂CO₃]/[HCO₃^-], in a solution containing both species which is maintained at pH = 7.00 by means of a buffer?

 

 

53. Given the following two solutions:  

                        0.200M NH₃(aq) with pK_b = 4.76       

                        0.200M HCl(aq) 

                 A mixture is made using 25.0 mL of the HCl and 50.0 mL of the NH₃.  What is its pH?

 

 

 

54. Given the following two solutions:  

                        0.200M HC₂H₃O₂(aq) with pK_a = 4.76   

                        0.200M NaOH(aq)                               

                 A mixture is made using 50.0 mL of the HC₂H₃O₂ and 25.0 mL of the NaOH.  What is its pH?

 

 

 

55. A mixture is made by mixing 50.0 mL of 0.200 M HCl and 50.0 mL of water.  What is its pH?

 

                      

 

 

56. Which is the most appropriate acid-base indicator to use in the titration of 0.100 molar acetic acid(aq) by 0.100 molar NaOH(aq)?  This is the titration of a weak acid by a strong base.  

 

 

 

57. Which is the most appropriate acid-base indicator to use in the titration of 0.100 molar ammonia solution by 0.100 molar HCl(aq)?  

 

                              

 

 

58. A solution is made by taking 100.0 mL of 4.0 × 10^-3 molar NaOH(aq) and mixing in 10.0 mL of 4.0 × 10^-2 molar HCl(aq).  A small portion was tested using thymol blue indicator, for which the pK_In = 8.8, and the colors are yellow for HIn, and blue for In^-.  What color would be observed? 

 

 

 

59. A solution is made by taking 100.0 mL of 2.0 × 10^-2 molar NaOH(aq) and mixing in 50.0 mL of 3.0

× 10^-2 molar HCl(aq).  A small portion was tested using tyrolian blue indicator, for which the pK_In = 12.5, and the colors are blue for HIn, and yellow for In^-.  What color would be observed as a result of the testing?

 

 

 

60. A solution is made by taking 2.42 grams of NaCl, 6.44 grams of NaClO₄, 3.81 grams of KNO₃, and 4.64 grams of KBr in enough water to make 500 mL of solution.  A portion was treated with thymol blue indicator, for which the pK_In = 8.8, and the colors are yellow for HIn, and blue for In^-.  What color should be obtained with the test portion?

 

 

 

61. The pH at the equivalence point of the titration of acetic acid solution by sodium hydroxide solution

is

 

 

 

62. The pH at the equivalence point of the titration of perchloric acid solution by sodium hydroxide solution is

 

 

 

63. The pH at the equivalence point of the titration of ammonia solution by hydrochloric acid solution is

 

                                      

 

 

64. 40.0 mL of 0.10 M HCl(aq) was added to 50.0 mL of 0.10 M NaOH(aq) and the mixture was stirred, then tested with a pH meter.  What pH should be obtained at 25.0 ^°C?

 

 

 

65. 50.0 mL of 0.10 M HCl(aq) was added to 40.0 mL of 0.10 M NaOH(aq) and the mixture was stirred, then tested with a pH meter.  What reading should be obtained for the pH at 25.0 ^°C?

 

 

 

66. 20.0 mL of 0.10 M H₂SO₄(aq) was added to 50.0 mL of 0.10 M NaOH(aq) and the mixture was stirred, then tested with a pH meter.  What reading should be obtained for the pH at 25.0 ^°C?

 

 

 

67. 40.0 mL of 0.20 M HCl(aq) was added to 50.0 mL of 0.20 M NaOH(aq) and the mixture was stirred, then tested with a pH meter.  What reading should be obtained for the pH at 25.0 ^°C?

 

 

68. 40.0 mL of 0.20 M HCl(aq) was added to 50.0 mL of 0.10 M NaOH(aq) and the mixture was stirred, then tested with a pH meter.  What pH should be obtained at 25.0 ^°C?

 

 

69. Given the following:  

                        0.20 M NaOH(aq)                               0.20 M HCl(aq) 

                  A mixture is made using 50.0 mL of the HCl and 50.0 mL of the NaOH.  What is its pH at 25.0 ^°C?

 

 

70. Given the following:  

                        0.20 M NaOH(aq)                               0.20 M HCl(aq) 

                  A mixture is made using 50.0 mL of the NaOH and 25.0 mL of the HCl.  What is its pH at 25.0 ^°C?

 

 

 

Fill in the Blanks

 

 

71. The pK_a of a weak acid is 6.50.  What is the value of K_a for this acid? ______

 

 

72. A 0.100 molar solution of a moderately strong monoprotic acid in water has a pH = 1.88.  Calculate the value of K_a, the acid ionization constant for this acid.  ______ 

                        

 

73. A solution of a molecular base B (0.50 M) and its salt BH⁺Cl^- (0.80 M) has a pH = 9.40.  What is the value of K_b for this base, B?  ________

 

 

74. A solution of a weak monoprotic acid, HA (0.50 molar), and its potassium salt, KA (0.75 molar), has a measured pH = 4.88.  What is the value of K_a for this acid?  _________ 

 

                              

 

75. Carbonic acid, H₂CO₃, is a diprotic acid with ionization constants:  K_a1 = 4.5 × 10^-7, K_a2 = 4.7 × 10^-11.  Calculate the pH of a 0.100 molar solution of Na₂CO₃.______ 

 

 

76. For H₃PO₃, which is actually a diprotic acid, K_a1 = 1.0 × 10^-2 and K_a2 = 2.6 × 10^-7.  Calculate the pH of a 0.100 molar solution of Na₂HPO₃.  ______

 

True and False

 

 

77. A relation involving acids and bases is, K_a + K_b = K_w.  ___

 

 

78. The pK_a is a reliable measure of the strength of an acid.  Strong acids have larger pK_a values than weak acids.  ___ 

 

 

79. A solution of sodium cyanide, NaCN, in water should be basic, not neutral or acidic.  ___ 

 

 

80. A solution of ammonium nitrate, NH₄NO₃ in water should be basic, not neutral or acidic.  ___ 

 

81. The titration of a strong acid by a strong base gives sharper and more well defined end points than titration of a weak acid by a weak base.  ___ 

Critical Thinking Questions

 

 

82. A student does a titration of NH₃(aq)(pK_b = 4.76) with HCl(aq).  When 100.0 mL of the HCl solution had been added to 500.0 mL of 0.300 M NH₃(aq), he measured the pH of the mixture and found it was 8.94.  What was the molarity of the HCl solution?  ______ 

 

 

83. A student does a titration of NH₃(aq) (pK_b = 4.76) with HCl(aq).  When 100.0 mL of the HCl solution had been added to 500.0 mL of 0.300 M NH₃(aq), she measured the pH of the mixture and found it was 9.54.  What was the molarity of the HCl solution?  ______ 

 

 

84. The pK_b for NH₃(aq) is 4.76.  What should be  the pH of a mixture containing 100.0 mL of 0.300 M NH₄Cl(aq), 100 mL of 0.300 M NH₃(aq), and 100 mL of 0.100 M NaOH?  ______

 

 

85. The pK_a for HC₂H₃O₂(aq) is 4.76.  What should be the pH of a mixture containing 100.0 mL of

0.300 M NaC₂H₃O₂(aq), 100 mL of 0.300 M HC₂H₃O₂(aq), and 100 mL of 0.100 M NaOH? ______ 

           

 

 

86. The pK_b for NH₃(aq) is 4.76.  What should be the pH of a mixture containing 100.0 mL of 0.300 M NH₄Cl(aq), 100 mL of 0.300 M NH₃(aq), and 100 mL of 0.100 M HCl(aq)?   ______ 

 

 

87. The pK_a for HC₂H₃O₂(aq) is 4.76.  What should be the pH of a mixture containing 100.0 mL of

0.300 M NaC₂H₃O₂(aq), 100 mL of 0.300 M HC₂H₃O₂(aq), and 100 mL of 0.100 M HCl(aq)?  ______ 

 

 

88. A mixture contains 400.0 mL of 1.00 M NH₃(aq), whose pK_b is 4.76, and  400.0 mL of 1.00 M NH₄Cl(aq).  200.0 mL of 0.500 M NaOH(aq) were added to the mixture.  What was the pH of the mixture before addition of the base and after addition of the base?  ______, ______ 

 

 

89. A mixture contains 400.0 mL of 1.00 M NH₃(aq), whose pK_b is = 4.76, and 400.0 mL of 1.00 M  NH₄Cl(aq).  200.0 mL of 0.500 M HCl(aq) were added to the mixture.  What is the pH of the buffer before addition of the acid and after addition of the acid?  ______, ______ 

 

 

90. A mixture contains benzoic acid, a monoprotic acid (K_a = 6.28 x 10^-5, 0.250 M), and sodium benzoate (0.400 M).  500 mL of this buffer were treated with 250 mL of an NaOH solution whose pH was

12.875.  What should the pH be after addition of the base?  _______

 

91. A student was instructed to prepare 2.00 liters of a buffer solution using 0.600 moles of sodium dihydrogen phosphate and 0.400 moles of sodium monohydrogen phosphate as the only solutes.  He got the instructions reversed somehow and made the solution using 0.400 moles of sodium dihydrogen phosphate and 0.600 moles of sodium monohydrogen phosphate.  He asked the postdoc student what to do.  "No problem", was the reply, "just add enough solid sodium hydroxide or 6.00 molar HCl(aq) to get the pH to where it should have been."  How many pH units off the target value was the pH of the solution the student prepared at first?  What should he add to the 2.00 liters of solution he now has, and in what quantity?  

                 ____________________________________________________________________________

      

 

 

92. A mixture contains benzoic acid, a monoprotic acid (K_a = 6.28 x 10^-5, 0.250 M), and sodium benzoate (0.400 moles/liter).  How many mL of a HCl(aq) solution whose pH is 0.523 should be added to 500 mL of this buffer to change its pH from what it is to pH = 4.20?   _______

 

Short answer

 

93.Potassium hydrogen phthalate, abbreviated KHP, is used to standardize NaOH solutions. A standard KHP solution is made by dissolving 2.12 grams of KHP in 100.00 mL of water. The KHP solution is then titrated with NaOH solution. It takes 23.12 mL of NaOH to reach the endpoint. What is the concentration of the NaOH? 

 

 

 

94. What is the pH of a 0.0100 M CH₃NH₂ solution? The K_b of methylamine is 4.4 × 10^-4. 

 

 

 

95. What is the pH of a 0.231 M solution of hydrazine, N₂H₄? The pK_b of hydrazine is 6.02. 

 

 

96. A solution of nitrous acid has a pH of 2.10. What is the concentration of nitrous acid in this solution? The K_a of nitrous acid is 7.1 x 10^-4. 

 

 

 

97. A solution of barbituric acid has a pH of 4.20. What is the concentration of barbituric acid in the solution? The pK_a of barbituric acid, HC₄H₃N₂O₃, is 4.01. 

 

 

 

98. What is the pH of a 0.300 M solution of sodium phenolate, NaC₆H₅O? The K_a of phenol is  1.3 x 10^-10. 

 

 

 

 

 

99. What is the pH of a 0.210 M solution of protonated pyridine, C₅H₅NH⁺? The K_b of pyridine is 1.5 ×

10^-9. 

 

 

100. A solution is made by dissolving 15.00 grams of acetic acid and 3.25 grams of sodium acetate in 500.00 mL of water. What is the pH of the solution? The K_a of acetic acid is 1.8 x 10^-5. 

 

101. Consider the following conjugate bases: Cl^-, Br^-, SO₄^2-, NO₂^-, ClO₄^-, ClO^-, and C₂O₄^2-. Which are weak bases and which are negligible bases? 

 

 

102. How many grams of sodium formate must be added to 100.00 mL of a 0.125M formic acid solution to achieve a pH of 4.90? The pK_a of formic acid is 3.74. 

 

 

 

103. Explain why organic bases such as pyridine, methylamine, and ethylamine, have conjugate acids that are positively charged. Explain in terms of the Lewis structures shown below. 

 

N

H2

 

                                    H3C NH2                H3C C NH2