Chem 121 Fritsch

Ch 16 and 17 Practice Problems - KEY The following problems are intended to provide you with additional practice in preparing for the exam. Questions come from the textbook, previous quizzes, previous exams, and other sources. A solutions manual is supplied in a separate document. 1. Calculate the pH of the solution during the titration of 15.00 mL of a 0.128 M benzoic acid (HC6H6O2, pKa = 4.20) with 0.167 M potassium hydroxide (KOH). a. Before adding any KOH.

b. After adding 4.41 mL of KOH.

Chem 121 Fritsch

c. At the equivalence point.

d. After addition17.54 mL of KOH.

Chem 121 Fritsch

2. Consider two solutions of formic acid HCHO2 (pKa = 3.74) which were prepared with concentrations of 0.19 M and 0.0034 M HCHO2. a. Which has a lower pH? b. Which has a higher percentage ionization?

Chem 121 Fritsch

3. For the following solutions, determine the [H3O+], [OH-], and the pH. a. A 0.35 M HNO2 solution (pKa = 3.14)

b. A 0.35 M HNO3 solution

c. A 0.00035 M HNO2 solution

Chem 121 Fritsch

d. A 0.16 M KOH solution

e. A 0.16 M KCN solution (pKb = 4.79)

f. A 0.16 M H2SO3 solution (pKa1 = 1.89, pKa2 = 7.21)

Chem 121 Fritsch

3. A 100 mL solution composed of 0.312 M chloroacetic acid (pKa = 2.85) and 0.269 M sodium chloroacetate was prepared. a. Calculate the pH of the solution.

Chem 121 Fritsch

b. Calculate the pH of the solution after adding 7.57 mL of 1.24 M HCl.

c. Calculate the pH of the solution after adding 7.57 mL of 0.76 M Ba(OH)2.

Chem 121 Fritsch

4. In the reaction below, identify the conjugate base of HSO4

is…. SO42 –

HSO4

(aq) + OH(aq) SO4

2(aq) + H2O(l)

5. Place the follow in order of increasing acidity (HC2H3O2, NH4Cl, HClO4, HCHO2) Increasingly strong acid

NH4Cl < HC2H3O2 < HCHO2 < HClO4 6. Morphine (C17H19NO3, molar mass = 285.34 g/mol) is used to manage pain and is often administered intravenously where the concentration is 1 mg morphine per mL of solution. In the General Chemistry laboratory you prepare an analogous solution from morphine and de-ionized water only. Calculate the pH of the solution you have prepared.

Chem 121 Fritsch

7. In the General Chemistry laboratory, your lab group combined 10.0 mL of 1.45 M HC2H3O2 (Ka = 1.8 x10-5) with 1.67 g of LiC2H3O2 (65.98 g/mol) in 25.00 mL volumetric flask and diluted it to the mark. a. Calculate the pH of the resulting solution.

Chem 121 Fritsch

b. Calculate the pH of the solution after you added 3.1 g of NaOH. (Assume no volume change)

c. Calculate the pH of the solution after you added 1 mL of 1.0 M HNO3.

Chem 121 Fritsch

8. Calculate the percent ionization for 100 mL of a 0.1 M benzoic acid solution with pH = 2.50. (Benzoic acid, HC7H5O2 is a white solid with MW = 122.0 g/mol)

HC7H5O2(aq) + H2O(l) C7H5O2-(aq) + H3O+

(aq) pKa = 4.20

9. Which of the following graphs shows the correct relationship between the molarity of a weak acid and its percent ionization?

A B C D

Chem 121 Fritsch

Answer:_____C____

10. Calculate the pH of the solution during the titration of 25.00 mL of a 0.349 M ethylamine (pKb = 3.37) with 0.186 M hydrochloric acid (HCl). a. Before adding any HCl.

b. After adding 24.57 mL of HCl.

Chem 121 Fritsch

c. After adding 46.91 mL of HCl.

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d. After addition 67.54 mL of HCl.

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11. Write Ka expressions for the following: a. HCN b. HCO3

- c. HBrO2 d. H3PO3

12. Calculate the pH of a solution containing 1.5 mL of pyridine (C5H5N, d = 0.982 g/mL, MW = 79.10 g/mol) Kb = 1.5 x 10-9) and 11.32 g of pyridinium chloride (HC5H5N+Cl-, MW = 115.56 g/mol) in a 50.0 mL volumetric flask and dissolved and diluted to the mark.

Chem 121 Fritsch

13. Choose which solution will have a lower pH when 0.15 moles of following solutes are is dissolved in enough water to make a 250 mL solution. a. H3PO4 or H3AsO4 b. HBrO or HBrO2 c. Co(NO3)2 or Co(NO3)3 d. FeCl3 or AlCl3

14. Identify the Lewis acid and Lewis Base among the reactants in each of the following reactions.

Chem 121 Fritsch

a. Ag+ + 2NH3 ↔ [Ag(NH3)2]+ b. AlCl3 + Cl- ↔ [AlCl4]- c. CN- + H2O ↔ HCN + OH- d. Ni2+ + 4CO ↔ [Ni(CO)4]2+ e. H2SO4 + NH3 ↔ HSO4

- + NH4+

15. Would the following combinations of chemicals yield buffer solutions? (Y/N) Justify your answer. HClO2 pKa = 1.96, NH4

+ pKa = 9.25

chemicals Buffer? (Y/N)

Justification

a. HClO2 with NaClO2 Y Weak acid (HClO2) with the salt of its conjugate base (NaClO2)

b. 10 mL of 1.0 M HClO2 with 5 mL of 1.0 M NaOH

Y Reaction of acid with NaOH makes NaClO2 in the presence of HClO2.

c. 10 mL of 1.0 M HClO2 with 5 mL of 1.0 M HCl

N Combining a weak acid with a strong acid yields no reaction

d. 10 mL of 1.0 M NaClO2 with 5 mL of 1.0 M HCl

Y Reaction acid with ClO2- makes HClO2 thus

both the acid and its conjugate base are present.

e. 10 mL of 1.0 M HCl with 5 mL of 1.0 M NaOH

N Reation between HCl and NaOH yields H2O and NaCl, neither of which is a weak acid/weak base.

Chem 121 Fritsch

f. 10 mL of 1.0 M NH3 with 5 mL of 1.0 M HCl

Y Reaction acid with NH3 makes NH4+ thus both

the acid and its conjugate base are present.

16. Strong acids like HCl fully dissociate into ions in aqueous solution. Can a weak acid like acetic acid (pKa = 4.74) dissociate to greater than 90 % in aqueous solution? A. Yes at any concentration. B. Yes only at low concentration (<1 x 10-2 M). C. Yes only at high concentration (>1 M). D. No, weak acids do not fully dissociate at any concentration. Answer:____B_____

17. Calculate the masses of acid and the sodium salt of its conjugate acid to make a 1.5 L of a 0.4 M buffer with pH 4.00.

Acid formula pKa

formic acid HCHO2 3.77

ammonium NH4+ 9.25

acetic acid HC2H3O2 4.74

chloroacetic acid HC2ClH2O2 2.85

fluoroacetic acid HC2FH2O2 2.60

ascorbic acid HC6H7O6 4.10

chlorous acid HClO2 1.96

phenol HC5H6O 10.00

benzoic acid HC7H5O2 4.20

hypochlorous acid HClO 7.54

hydrofluoric acid HF 3.25

nitrous acid HNO2 3.35

Chem 121 Fritsch

18. Like water, other compounds undergo autoionization. For methanol (CH3OH), the autoionization constant (Kmet) is 2.0 x10-17 at 25 °C. Calculate the [CH3O-] and [CH3OH2

+] in pure methanol at 25 °C.

Chem 121 Fritsch

19. For each of the following identify the acid, base, conjugate acid, and conjugate base. a. HC7H4NO4(aq) + H2O(l) ↔C7H4NO4

-(aq) + H3O+

(aq) b. NH4Cl(aq) + H2O(l) ↔NH3(aq) + H3O+

(aq) + Cl-(aq)

c. KCN(aq) + HCHO2(aq) ↔HCN(aq) + K+(aq) + CHO2

-(aq)

20. Acid strength decreases in the series HI > HSO4 > HF > HCN. Which of the following anions is the

weakest base?

A. I

B. SO42

C. F

D. CN E. It cannot be determined from the available information. Answer:____A____