Ch 16 and 17 Practice Problems 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. c. At the equivalence point. d. After addition17.54 ml of KOH. 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? 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 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)
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. 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. 4. In the reaction below, identify the conjugate base of HSO 4 is. HSO 4 (aq) + OH (aq) SO 4 2 (aq) + H 2O (l) 5. Place the follow in order of increasing acidity (HC 2H 3O 2, NH 4Cl, HClO 4, HCHO 2) Increasingly strong acid 6. Morphine (C 17H 19NO 3, 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. 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. 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. 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 Answer: 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. c. After adding 46.91 ml of HCl. d. After addition 67.54 ml of HCl. 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.
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 Chem 121 14. Identify the Lewis acid and Lewis Base among the reactants in each of the following reactions. a. Ag + + 2NH3 [Ag(NH3)2] + b. AlCl3 + Cl- [AlCl4] - c. CN - + H2O HCN + OH - d. Ni 2+ + 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 a. HClO2 with NaClO2 b. 10 ml of 1.0 M HClO2 with 5 ml of 1.0 M NaOH c. 10 ml of 1.0 M HClO2 with 5 ml of 1.0 M HCl d. 10 ml of 1.0 M NaClO2 with 5 ml of 1.0 M HCl e. 10 ml of 1.0 M HCl with 5 ml of 1.0 M NaOH f. 10 ml of 1.0 M NH3 with 5 ml of 1.0 M HCl Buffer? (Y/N) Justification
16. Strong acids like HCl fully dissociate into ions in aqueous solution. Can a weak acid like acetic acid (pk a = 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: 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 pk a formic acid HCHO 2 3.77 ammonium + NH 4 9.25 acetic acid HC 2H 3O 2 4.74 chloroacetic acid HC 2ClH 2O 2 2.85 fluoroacetic acid HC 2FH 2O 2 2.60 ascorbic acid HC 6H 7O 6 4.10 chlorous acid HClO 2 1.96 phenol HC 5H 6O 10.00 benzoic acid HC 7H 5O 2 4.20 hypochlorous acid HClO 7.54 hydrofluoric acid HF 3.25 nitrous acid HNO 2 3.35 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. 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 > HSO 4 > HF > HCN. Which of the following anions is the weakest base? A. I B. 2 SO 4 C. F D. CN E. It cannot be determined from the available information. Answer: