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AP Chem Topic#11

Acid/Base Equilibria Test Review WS

Multiple Choice

Use the following answers for questions 1 - 3.

(A) F (B) S (C) Mg (D) Ar (E) Mn

1. Forms monatomic ions with 2- charge in solutions

2. Forms a compound having the formula KXO4

3. Forms oxides that are common air pollutants and that yield acidic solution in water.

Use the following answers for questions 4 - 7.

(A) Hydrofluoric acid (B) Carbon dioxide (C) Aluminum hydroxide (D) Ammonia (E) Hydrogen peroxide

4. Is a good oxidizing agent

5. Is used to etch glass chemically

6. Is used extensively for the production of fertilizers

7. Has amphoteric properties.

8. The pH of 0.1-molar ammonia is approximately

A) 1

B) 4

C) 7

D) 11

E) 14

9. Each of the following can act as both a Brönsted acid and a Brönsted base EXCEPT

A) HCIO31-

B) H2PO41-

C) NH41+

D) H2O

E) HS1-

10. Which, if any, of the following species is in the greatest concentration in a 0.100-molar solution of H2SO4 in water?

A) H2SO4 molecules

B) H3O+ ions

C) HSO4¯ ions

D) SO42¯ ions

E) All species are in equilibrium and therefore have the same concentrations.

|Acid |Acid Dissociation Constant, Ka |

|H3PO4 |7x10-3 |

|H2PO41- |8x10-8 |

|HPO42- |5x10-13 |

11. On the basis of the information above, a buffer with a pH = 9 can best be made by using

(A) pure NaH2PO4 (B) H3PO4 + H2PO41- (C) H2PO41- + PO43- (D) H2PO41- + HPO42- (E) HPO42- + PO43-

12. The net ionic equation for the reaction that occurs during the titration of nitrous aicd with sodium hydroxide is

A) HNO2 + Na+ + OH1- → NaNO2 + H2O

B) HNO2 + NaOH → Na+ + NO2¯ + H2O

C) H+ + OH1- → H2O

D) HNO2 + H2O → NO21- + H3O+

E) HNO2 + OH¯ → NO21- + H2O

13. If the acid dissociation constant, Ka, for an acid HA is 8x10-4 at 25 °C, what percent of the acid is dissociated in a 0.50M solution of HA at 25 °C?

A) 0.08%

B) 0.2%

C) 1%

D) 2%

E) 4%

Use these answers for questions 14 - 16.

(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 = 1.8x10-5; NH3 = 1.8x10-5; H2CO3; K1 = 4x10-7; H2CO3; K2 = 4x10-11

14. A solution prepared to be initially 1M in NaCl and 1M in HCl.

15. A solution prepared to be initially 1M in Na2CO3 and 1M in CH3COONa

16. A solution prepared to be initially 0.5M in CH3COOH and 1M in CH3COONa

17. 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.

18. All of the following species can function as Brönsted-Lowry bases in solution EXCEPT

A) H2O

B) NH3

C) S2-

D) NH4+

E) HCO31-

19. As the number of oxygen atoms increases in any series of oxygen acids, such as HXO, HXO2, HXO3, ...., which of the following is generally true?

A) The acid strength varies unpredictably.

B) The acid strength decreases only if X is a nonmetal.

C) The acid strength decreases only if X is a metal.

D) The acid strength decreases whether X is a nonmetal or a metal.

E) The acid strength increases.

20. Which of the following is the correct equilibrium expression for the hydrolysis of CO32¯ ?

A) K = [HCO3¯ ] / ( [CO32¯ ] [H3O+] )

B) K = ( [HCO3¯] [OH¯] ) / [CO32¯]

C) K = ( [CO32¯ ] [OH¯] ) / [HCO3¯]

D) K = [CO32¯ ] / ( [CO2] [OH¯]2 )

E) K = ( [CO32¯ ] [H3O+] ) / [HCO3¯]

21. H2PO41- + HBO32- ↔ HPO42- + H2BO31-

The equilibrium constant for the reaction represented by the equation above is greater than 1.0. Which of the following gives the correct relative strengths of the acids and bases in the reaction?

| |Acids | |Bases |

|(A) |H2PO41-> H2BO31- |and |HBO32- > HPO42- |

|(B) |H2BO31- > H2PO41- |and |HBO32- > HPO42- |

|(C) |H2PO41- > H2BO31- |and |HPO42- > HBO32- |

|(D) |H2BO31- > H2PO41- |and |HPO42- > HBO32- |

|(E) |H2PO41- = H2BO31- |and |HPO42- = HBO32- |

22. A 0.20M solution of a weak monoprotic acid, HA, has a pH of 3.00. The ionization constant of this acid is

A) 5.0 x 10-7

B) 2.0 x 10-7

C) 5.0 x 10-6

D) 5.0 x 10-3

E) 2.0 x 10-3

23. Equal volumes of 0.10M H3PO4 and 0.20M KOH are mixed. After equilibrium is established, the type of ion an solution in largest concentration, other than the K+ ion, is

A) H2PO41-

B) HPO42-

C) PO43-

D) OH1-

E) H3O+

24. HSO41- + H2O ↔ H3O+ + SO42-

In the equilibrium represented above, the species that act as bases include which of the following?

I. HSO41- II. H2O III. SO42-

A) II only

B) III only

C) I and II

D) I and III

E) II and III

25. H2C2O4 + 2 H2O → 2 H3O+ + C2O42-

Oxalic acid, H2C2O4, is a diprotic acid with K1 = 5.36x10-2 and K2 = 5.3x10-5. For reaction above, what is the equilibrium constant?

A) 5.36x10-2

B) 5.3x10-5

C) 2.8x10-6

D) 1.9x10-7

E) 1.9x10-13

26. Which of the following acids can be oxidized to form a stronger acid?

A) H3PO4

B) HNO3

C) H2CO3

D) H3BO3

E) H2SO3

27. A 1-molar solution of which of the following salts has the highest pH ?

A) NaNO3

B) Na2CO3

C) NH4Cl

D) NaHSO4

E) Na2SO4

28. What is the pH of a 1.0x10-2-molar solution of HCN? (For HCN, Ka = 4.0x10-10.)

A) 10

B) Between 7 and 10

C) 7

D) Between 4 and 7

E) 4

29. A solution of calcium hypochlorite, a common additive to swimming-pool water, is

(A) basic because of the hydrolysis of the OCl1- ion

(B) basic because Ca(OH)2 is a weak and insoluble base

(C) neutral if the concentration is kept below 0.1 molar

(D) acidic because of the hydrolysis of the Ca2+ ions

(E) acidic because the acid HOCl is formed

30. 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

B) 0.800 M

C) 0.640 M

D) 0.600 M

E) 0.400 M

31. What volume of 0.150-molar HCl is required to neutralize 25.0 millilters of 0.120-molar Ba(OH)2?

A) 20.0 mL

B) 30 0 mL

C) 40.0 mL

D) 60.0 mL

E) 80.0 mL

32. 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

B) II only

C) I and III only

D) II and III only

E) I, II, and III

33. A student pipetted five 25.00-milliliter samples of hydrochloric acid and transferred each sample to an Erlenmeyer flask, diluted it with distilled water, and added a few drops of phenolphthalein to each. Each sample was then titrated with a sodium hydroxide solution to the appearance of the first permanent faint pink color. The following results were obtained. Volumes of NaOH Solution

First Sample..................35.22 mL

Second Sample..............36.14 mL

Third Sample.................36.13 mL

Fourth Sample..............36.15 mL

Fifth Sample..................36.12 mL

Which of the following is the most probable explanation for the variation in the student's results?

(A) The burette was not rinsed with NaOH solution.

(B) The student misread a 5 for a 6 on the burette when the first sample was titrated.

(C) A different amount of water was added to the first sample.

(D) The pipette was not rinsed with the HCI solution.

(E) The student added too little indicator to the first sample.

34. 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) OH1- acts as a catalyst for the decomposition of phenolphthalein

(E) phenolphthalein is involved in the rate-determining step of the reaction between H3O+ and OH1-

Problems

1. NH3(aq) + H2O(l) ↔ NH4+(aq) + OH1-(aq)

In aqueous solution, ammonia reacts as represented above. In 0.0180M NH3 (aq) at 25ºC, the hydroxide ion concentration, [OH1-] is 5.60x10-4M. In answering the following, assume that temperature is constant at 25ºC and that volumes are additive.

A) Write the equilibrium-constant expression for the reaction represented above.

B) Determine the pH of 0.0180M 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).

a. Determine the volume of 0.0120M HCl(aq) that was added to reach the equivalence point.

b. Determine the pH of the solution in the flask after a total of 15.0 mL of 0.0120M HCl(aq) was added.

c. Determine the pH of the solution in the flask after a total of 40.0 mL of 0.0120M HCl(aq) was added.

2. An approximately 0.1-molar solution of NaOH is to be standardized by titration. Assume that the following materials are available.

Clean, dry 50 mL buret

250 mL Erlenmeyer flask

Wash bottle filled with distilled water

Analytical balance

Phenolphthalein indicator solution

Potassium hydrogen phthalate, KHP, a pure solid monoprotic acid (to be used as the primary standard)

A) Briefly describe the steps you would take, using the materials listed above, to standardize the NaOH solution.

B) Describe (i.e., set up) the calculations necessary to determine the concentration of the NaOH solution.

C) After the NaOH solution has been standardized, it is used to titrate a weak monoprotic acid, HX. The equivalence point is reached when 25.0mL of NaOH solution has been added. In the space provided at the right, sketch the titration curve, showing the pH changes that occur as the volume of NaOH solution added increases from 0 to 35.0mL. Clearly label the equivalence point on the curve.

D) Describe how the value of the acid-dissociation constant, Ka, for the weak acid HX could be determined from the titration curve in part (c).

Answer Key

MC

1. B

2. E

3. B

4. E

5. A

6. D

7. C

8. D

9. C

10. B

11. D

12. E

13. E

14. A

15. D

16. B

17. C

18. D

19. E

20. B

21. A

22. C

23. B

24. E

25. C

26. E

27. B

28. D

29. A

30. B

31. C

32. C

33. C

34. B

Problems

1.

(a) Kb =

(b) pOH = -log(5.60x10–4) = 3.252

pH = 14 – pOH = 10.748

(c) Kb = [pic] = 1.80x10–5

(d) x 100% = 3.11%

(e) (a) NaVa = NbVb

(0.0120N)(Va) = (0.0180N)(20.0mL)

Va = 30.0 mL

(b) at 15.0 mL it is half-titrated and [NH41+] = [NH3], then the Kb = [OH1-] = 1.80x10-5.

pOH = -log(1.80x10-5) = 4.745

pH = 14 – 4.75 = 9.255

(c) at 40.0 mL, there is an excess of 10.0 mL of HCl past equivalence point,

(10.0 mL)(0.0120 M) = 0.120 mmol H1+ ions

= 0.00200M =[H+]

pH = -log(0.00200) = 2.70

2.

(A)

● exactly mass a sample of KHP in the Erlenmeyer flask and add distilled water to dissolve the solid.

● add a few drops of phenolphthalein to the flask.

● rinse the buret with the NaOH solution and fill.

● record starting volume of base in buret.

● with mixing, titrate the KHP with the NaOH solution until it just turns slightly pink.

● record end volume of buret.

● repeat to check your results.

A) [pic] = moles of KHP

since KHP is monoprotic, this is the number of moles of NaOH

[pic] = molarity of NaOH

B) [pic]

C) from the titration curve, at the 12.5 mL volume point, the acid is half-neutralized and the pH = pKa. Ka = 10pKa

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