Chapter 18 Practice – Buffers, Titration, Ksp
AP Chemistry
1984
63. On the basis of the information below, a buffer with a pH = 9 can best be
made by using
(A) pure NaH2PO4
(B) H3PO4 + H2PO4(C) H2PO4- + PO43(D) H2PO4- + HPO42(E) HPO42- + PO43-
Acid
Acid Dissociation
Constant, Ka
H3PO4
7 × 10-3
H2PO4-
8 × 10-8
HPO42-
5 × 10-13
64. The net ionic equation for the reaction that occurs during titration of nitrous acid with sodium hydroxide is
(A) HNO2 + Na+ + OH- → NaNO2 + H2O
(B) HNO2 + NaOH → Na+ + NO2- + H2O
(C) H+ + OH- → H2O
(D) HNO2 + H2O → NO2- + H3O+
(E) HNO2 + OH- → NO2- + H2O
74. How many moles of NaF must be dissolved in 1.00 liter of a saturated solution of PbF2 at 25°C to reduce the
[Pb2+] to 1 × 10- 6 molar? (Ksp of PbF2 at 25°C = 4.0 × 10- 8)
(A) 0.020 mole
(D) 0.20 mole
(B) 0.040 mole
(E) 0.40 mole
(C) 0.10 mole
81. What is the net ionic equation for the reaction that occurs when aqueous copper(II) sulfate is added to excess
6-molar ammonia?
(A) Cu2+ + SO42- + 2 NH4+ + 2 OH- → (NH4)2SO4 + Cu(OH)2
(B) Cu2+ + 4 NH3 + 4 H2O → Cu(OH) + 4 NH4+
(C) Cu2+ + 2 NH3 + 2 H2O → Cu(OH)2 + 2 NH4+
(D) Cu2+ + 4 NH3 → Cu(NH3)42+
(E) Cu2+ + 2 NH3 + H2O → CuO + 2 NH4+
1989
Use these answers for questions 8 - 10.
(A) a solution with a pH less than 7 that is not a buffer solution
(B) a buffer solution with a pH between 4 and 7
(C) a buffer solution with a pH between 7 and 10
(D) a solution with a pH greater than 7 that is not a buffer solution
(E) a solution with a pH of 7
−
Ionization Constants
CH3COOH = 1.8 × 10- 5
H2CO3; K1= 4 ×10-7
-5
NH3 = 1.8 × 10
H2CO3; K2 = 4 × 10-11
8. A solution prepared to be initially 1 M in NaCl and 1 M in HCl.
9. A solution prepared to be initially 1 M in Na2CO3 and 1 M in CH3COONa
10. A solution prepared to be initially 0.5 M in CH3COOH and 1 M in CH3COONa
19. In the titration of a weak acid of unknown concentration with a standard solution of a strong base, a pH
meter was used to follow the progress of the titration. Which of the following is true for this experiment?
(A) The pH is 7 at the equivalence point.
(B) The pH at the equivalence point depends on the indicator used.
(C) The graph of pH versus volume of base added rises gradually at first and then much more rapidly.
(D) The graph of pH versus volume of base added shows no sharp rise.
(E) The [H+] at the equivalence point equals the ionization constant of the acid.
1
35. When phenolphthalein is used as the indicator in a titration of an HCl solution with a solution of NaOH, the
indicator undergoes a color change from clear to red at the end point of the titration. This color change
occurs abruptly because
(A) phenolphthalein is a very strong acid that is capable of rapid dissociation
(B) the solution being titrated undergoes a large pH change near the end point of the titration
(C) phenolphthalein undergoes an irreversible reaction in basic solution
(D) OH- acts as a catalyst for the decomposition of phenolphthalein
.
(E) phenolphthalein is involved in the rate-determining step of the reaction between H3O+ and OH52. The test for the presence of Ag+ in an unknown solution involves the treatment of the solvent-ammonia
complex with dilute hydrochloric acid. The appearance of a white precipitate at this point indicates the
presence of silver ion in the original sample. The net ionic equation that represents this test is
(A) Ag(NH3)2+ + 2 H+  Ag(s) + 2 NH4+
(B) Ag(NH3)2+ + Cl-  AgCl(s) + 2 NH3
(C) Ag(NH3)2+ + 2 HCl  AgCl(s) + 2 NH4+ + Cl(D) Ag(NH3)2+ + Cl-  Ag(NH3)2Cl(s)
(E) Ag(NH3)2+ + 2 H+ + Cl-  AgCl(s) + 2 NH4+
66. MnS(s) + 2 H+  Mn2+ + H2S(g)
At 25°C the solubility product constant, Ksp, for MnS is 5 × 10- 15 and the acid dissociation constants Kl
and K2 for H2S are 1 × 10-7 and 1 × 10- 13, respectively. What is the equilibrium constant for the reaction
represented by the equation above at 25°C?
(A) 1 × 10- 13/5 × 10 - 15
(D) 5 × 10-15/1 × 10- 20
(B) 5 × 10- 15/ 1 × 10 - 7
(E) 1 × 10- 20/ 5 × 10- 15
-7
- 20
(C) 1 × 10 /5 × 10
69. A white solid is observed to be insoluble in water, insoluble in excess ammonia solution, and soluble in
dilute HCl. Which of the following compounds could the solid be?
(A) CaCO3
(D)AgCl
(B) BaSO4
(E) Zn(OH)2
(C) Pb(NO3)2
1994
16. Commercial vinegar was titrated with NaOH solution to determine the content of acetic acid, HC2H3O2. For
20.0 milliliters of the vinegar, 32.0 milliliters of 0.500-molar NaOH solution was required. What was the
concentration of acetic acid in the vinegar if no other acid was present?
(A) 1.60 M
(D) 0.600 M
(B) 0.800 M
(E) 0.400 M
(C) 0.640 M
53. If 87 grams of K2SO4 (molar mass 174 grams) is dissolved in enough water to make 250 milliliters of
solution, what are the concentrations of the potassium and the sulfate ions?
[K+]
[SO42-]
(A) 0.020 M 0.020 M
(B) 1.0 M
2.0 M
(C) 2.0 M
1.0 M
(D) 2.0 M
2.0 M
(E) 4.0 M
2.0 M
55. What volume of 0.150-molar HCl is required to neutralize 25.0 mL of 0.120-molar Ba(OH)2?
(A) 20.0 mL
(D) 60.0 mL
(B) 30.0 mL
(E) 80.0 mL
(C) 40.0 mL
2
65. Barium sulfate is LEAST soluble in a 0.01-molar solution of which of the following?
(A) Al2(SO4)3
(D) NH3
(B) (NH4)2SO4
(E) BaCl2
(C) Na2SO4
69. Correct procedures for a titration include which of the following?
I. Draining a pipet by touching the tip to the side of the container used for the titration
II. Rinsing the buret with distilled water just before filling it with the liquid to be titrated
III. Swirling the solution frequently during the titration
(A) I only
(D) II and III only
(B) II only
(E) I, II, and III
(C) I and III only
70. To determine the molar mass of a solid monoprotic acid, a student titrated a weighed sample of the acid with
standardized aqueous NaOH. Which of the following could explain why the student obtained a molar mass
that was too large?
I. Failure to rinse all acid from the weighing paper into the titration vessel
II. Addition of more water than was needed to dissolve the acid
III. Addition of some base beyond the equivalence point
(A) I only
(D) II and III only
(B) III only
(E) I, II, and III
(C) I and II only
1999
Questions 9-12 refer to aqueous solutions containing 1: 1 mole ratios of the following pairs of substances.
Assume all concentrations are 1 M. A choice may be used once, more than once, or not at all.
(A) NH3 and NH4Cl
(D) NaOH and NH3
(B) H3PO4 and NaH2PO4
(E) NH3 and HC2H3O2 (acetic acid)
(C) HCl and NaCl
9. The solution with the lowest pH
10. The most nearly neutral solution
11. A buffer at a pH > 8
12. A buffer at a pH < 6
33. A 1.0 L sample of an aqueous solution contains 0.10 mol of NaCl and 0.10 mol of CaCl2. What is the
minimum number of moles of AgNO3 that must be added to the solution in order to precipitate all of the Clas AgCl(s)? (Assume that AgCl is insoluble.)
(A) 0.10 mol
(C) 0.30 mol
(E) 0.60 mol
(B) 0.20 mol
(D) 0.40 mol
43. A sample of 61.8 g of H3BO3, a weak acid, is dissolved in 1,000 g of water to make a 1.0-molal solution.
Which of the following would be the best procedure to determine the molarity of the solution? (Assume no
additional information is available.)
(A) Titration of the solution with standard acid
(B) Measurement of the pH with a pH meter
(C) Determination of the boiling point of the solution
(D) Measurement of the total volume of the solution
(E) Measurement of the specific heat of the solution
3
65. Which of the following compounds is NOT appreciably soluble in water but is soluble in dilute hydrochloric
acid?
(A) Mg(OH)2(s)
(D) (NH4)2SO4(s)
(B) (NH4)CO3(s)
(E) Sr(NO3)2(s)
(C) CuSO4(s)
70. When 100 mL of 1.0 M Na3PO4 is mixed with 100 mL of 1.0 M AgNO3, a yellow precipitate forms and
[Ag+] becomes negligibly small. Which of the following is a correct listing of the ions remaining in solution
in order of increasing concentration?
(A) [PO43–] < [NO3–] < [Na+]
(D) [Na+] < [NO3–] < [PO43–]
(B) [PO43–] < [Na+] < [NO3–]
(E) [Na+] < [PO43-] < [NO3–]
(C) [NO3 –] < [PO43–] < [Na+]
71. In a qualitative analysis for the presence of Pb2+, Fe2+, and Cu2+ ions in aqueous solution, which of the
following will allow the separation of Pb2+ from the other ions at room temperature?
(A) Adding dilute Na2S(aq) solution
(B) Adding dilute HCl(aq) solution
(C) Adding dilute NaOH(aq) solution
(D) Adding dilute NH3(aq) solution
(E) Adding dilute HNO3(aq) solution
2002
30. At 25°C, aqueous solutions with a pH of 8 have a hydroxide ion concentration, [OH-], of
(A) 1 × 10-14 M
(B) 1 × 10-8 M
(C) 1 × 10-6 M
(D) 1 M
(E) 8 M
Questions 33-34
The graph below shows the titration curve that results when 100.
mL of 0.0250 M acetic acid is titrated with 0.100 M NaOH.
33. Which of the following indicators is the best choice for this
titration?
Indicator
pH Range of Color Change
(A) Methyl orange
3.2 - 4.4
(B) Methyl red
4.8 - 6.0
(C) Bromothymol blue
6.1 - 7.6
(D) Phenolphthalein
8.2 - 10.0
(E) Alizarin
11.0 - 12.4
34. What part of the curve corresponds to the optimum buffer
action for the acetic acid/acetate ion pair?
(A) Point V
(C) Point Z
(B) Point X
(D) Along all of section WY
(E) Along all of section YZ
61. How can 100. mL of sodium hydroxide solution with a pH of 13.00 be converted to a sodium hydroxide
solution with a pH of 12.00 ?
a) By diluting the solution with distilled water to a total volume of 108 mL
b) By diluting the solution with distilled water to a total volume of 200 mL
c) By diluting the solution with distilled water to a total volume of 1.00 L
d) By adding 100. mL of 0.10 M HCl
e) By adding 100. mL of 0.10 M NaOH
4
63. Mixtures that would be considered buffers include which of the following?
I. 0.10 M HCl + 0.10 M NaCl
II. 0.10 M HF + 0.10 M NaF
III. 0.10 M HBr + 0.10 M NaBr
(A) I only
(C) III only
(B) II only
(D) I and II
(E) II and III
64. Ascorbic acid, H2C6H6O6(s), is a diprotic acid with K1 = 7.9 × 10-5 and K2 = 1.6 × 10-12. In a 0.005 M
aqueous solution of ascorbic acid, which of the following species is present in the lowest concentration?
(A) H2O(l)
(D) HC6H6O6-(aq)
+
(B) H3O (aq)
(E) C6H6O62-(aq)
(C) H2C6H6O6(aq)
72. A colorless solution is divided into three samples. The following tests were performed on samples of the
solution.
Sample
Test
Observation
1
Add H+(aq)
No change
2
Add NH3(aq)
No change
23
Add SO4 (aq)
No change
Which of the following ions could be present in the solution at a concentration of 0.10 M?
(A) Ni2+(aq)
(D) Na+(aq)
(B) Al3+(aq)
(E) CO32-(aq)
2+
(C) Ba (aq)
75. In a saturated solution of Zn(OH)2 at 25°C, the value of [OH-] is 2.0 × 10-6 M. What is the value of the
solubility-product constant, Ksp, for Zn(OH)2 at 25°C?
(A) 4.0 × 10-18
(D) 4.0 × 10-12
(B) 8.0 × 10-18
(E) 2.0 × 10-6
-17
(C) 1.6 × 10
1999
+
NH3(aq) + H2O(l)  NH4 (aq) + OH–(aq)
In aqueous solution, ammonia reacts as represented above. In 0.0180 M NH3(aq) at 25ºC, the hydroxide ion concentration,
–
–4
[OH ] is 5.60×10 M. In answering the following, assume that temperature is constant at 25ºC and volumes are additive.
(a) Write the equilibrium-constant expression for the reaction represented above.
(b) Determine the pH of 0.0180 M NH3(aq).
(c) Determine the value of the base ionization constant, Kb, of NH3(aq).
(d) Determine the percent ionization of NH3 in 0.0180 M NH3(aq).
(e) In an experiment, a 20.0 mL sample of 0.0180 M NH3(aq) was placed in a flask and titrated to the equivalence point
and beyond using 0.0120 M HCl(aq).
(i) Determine the volume of 0.0120 M HCl(aq) that was added to reach the equivalence point.
(ii) Determine the pH of the solution in the flask after a total of 15.0 mL of 0.0120 M HCl(aq) was added.
(iii) Determine the pH of the solution in the flask after a total of 40.0 mL of 0.0120 M HCl(aq) was added.
5
2001
1. Answer the following questions relating to the solubility of the chlorides of silver and lead.
a) At 10°C, 8.9 × 10-5 g of AgCl(s) will dissolve in 100. mL of water.
i) Write the equation for the dissociation of AgCl(s) in water.
ii) Calculate the solubility, in mol L-1, of AgCl(s) in water at 10°C.
iii) Calculate the value of the solubility-product constant, Ksp, for AgCl(s) at 10°C.
b) At 25°C, the value of Ksp for PbC12(s) is 1.6 × 10-5 and the value of Ksp for AgCl(s) is 1.8 × 10-10.
i) If 60.0 mL of 0.0400 M NaCl(aq) is added to 60.0 mL of 0.0300 M Pb(NO3)2(aq), will a precipitate
form? Assume that volumes are additive. Show calculations to support your answer.
ii) Calculate the equilibrium value of [Pb2+(aq)] in 1.00 L of saturated PbCl2 solution to which 0.250
mole of NaCl(s) has been added. Assume that no volume change occurs.
iii) If 0.100 M NaCl(aq) is added slowly to a beaker containing both 0.120 M AgNO3(aq) and 0.150 M
Pb(NO3)2(aq) at 25°C, which will precipitate first, AgCl(s) or PbCl2(s)? Show calculations to
support your answer.
2002 (Form B)
8. The graph below shows the result of the titration of a
25 mL sample of a 0.10 M solution of a weak acid,
HA, with a strong base, 0.10 M NaOH.
(a) Describe two features of the graph above that
identify HA as a weak acid.
(b) Describe one method by which the value of the
acid-dissociation constant for HA can be
determined using the graph above.
(c) On the graph above, sketch the titration curve
that would result if 25 mL of 0.10 M HCl were
used instead of 0.10 M HA.
(d) A 25 mL sample of 0.10 M HA is titrated with 0.20 M NaOH.
(i) What volume of base must be added to reach the equivalence point?
(ii) The pH at the equivalence point for this titration is slightly higher than the pH at the equivalence
point in the titration using 0.10 M NaOH. Explain.
6
2004
1. Answer the following questions relating to the solubilities of two silver compounds, Ag2CrO4 and Ag3PO4.
Silver chromate dissociates in water according to the equation shown below.
Ag2CrO4(s)  2 Ag+(aq) + CrO42-(aq) Ksp = 2.6 × 10-12 at 25°C
a) Write the equilibrium-constant expression for the dissolving of Ag2CrO4(s).
b) Calculate the concentration, in mol L-1, of Ag+(aq) in a saturated solution of Ag2CrO4 at 25°C.
c) Calculate the maximum mass, in grams, of Ag2CrO4 that can dissolve in 100. mL of water at 25°C.
d) A 0.100 mol sample of solid AgNO3 is added to a 1.00 L saturated solution of Ag2CrO4. Assuming no
volume change, does [CrO42-] increase, decrease, or remain the same? Justify your answer.
In a saturated solution of Ag3PO4 at 25°C, the concentration of Ag+(aq) is 5.3 × 10-5 M. The equilibrium
constant expression for the dissolving of Ag3PO4(s) in water is shown: Ksp = [Ag+]3[PO43-]
e) Write the balanced equation for the dissolving of Ag3PO4 in water.
f) Calculate the value of Ksp for Ag3PO4 at 25°C.
g) A 1.00 L sample of saturated Ag3PO4 solution is allowed to evaporate at 25°C to a final volume of 500.
mL. What is [Ag+] in the solution? Justify your answer.
7
2009 Form B
1. A pure 14.85 g sample of the weak base ethylamine, C2H5NH2 , is dissolved in enough distilled water to
make 500. mL of solution.
a) Calculate the molar concentration of the C2H5NH2 in the solution.
The aqueous ethylamine reacts with water according to the equation below.
C2H5NH2 (aq) + H2O(l)  C2H5NH3+(aq) + OH-­‐(aq)
b) Write the equilibrium-constant expression for the reaction between C2H5NH2 (aq) and water.
c) Of C2H5NH2 (aq) and C2H5NH3+(aq) , which is present in the solution at the higher concentration at
equilibrium? Justify your answer.
d) A different solution is made by mixing 500. mL of 0.500 M C2H5NH2 with 500. mL of 0.200 M HCl.
Assume that volumes are additive. The pH of the resulting solution is found to be 10.93.
(i) Calculate the concentration of OH-­‐ (aq) in the solution.
(ii) Write the net-ionic equation that represents the reaction that occurs when the C2H5NH2 solution is
mixed with the HCl solution.
(iii) Calculate the molar concentration of the C2H5NH3+(aq) that is formed in the reaction.
(iv) Calculate the value of Kb for C2H5NH2.
8