Acid-Base Equilibria

Acid-Base Equilibria 1. Classify each of the following species as an acid, a base, or amphoteric in aqueous solution: (a) H 2 O; (b) CH 3 CH 2 ; (c) PO 4 3 ; (d) C 6 H 5 NH 3 2. Write the proton transfer equilibria for the following acids in aqueous solution and identify the conjugate acidbase pairs in each one: (a) H 2 SO 4 ; (b) C 6 H 5 NH 3 (anilinium ion); (c) H 2 PO 4 (dihydrogen phosphate ion); HCOOH (formic acid); (e) NH 2 NH 3 (hydrazinium ion). 3. Identify (a) the Bronsted acid and base in the following reaction, and (b) the conjugate base and acid formed: HNO 3 (aq) HPO 4 (aq) NO 3 (aq) H 2 PO 4 (aq) 4. The value of K W for water at body temperature 37 C is 2.5 10 14. (a) What is the molarity of H 3 O ions and the ph of neutral water at 37 C? (b) What is the molarity of OH in neutral water at 37 C? 5. Calculate the nominal molarity of HCl and the molarities of H 3 O, Cl, and OH in an aqueous solution that contains 0.48 mol of HCl in 500 ml of solution. 6. Calculate the nominal molarity of Ba(OH) 2 and the molarities of Ba 2, OH, and H 3 O in an aqueous solution that contains 0.50 g of Ba(OH) 2 in 100 ml of solution. 7. The ph of several solutions was measured in the research laboratories of a food company. Convert each of the following ph values to the molarity of H 3 O ions: (a) 3.3 (the ph of sour orange juice); (b) 6.7 (the ph of a saliva sample); (c) 4.4 (the ph of beer); (d) 5.3 (the ph of a coffee sample). (e) List the samples in order of increasing acidity. 8. Calculate the K a values of the following acids: (a) phosphoric acid (H 3 PO 4 ), pk a1 = 2.12; (b) phosphorous acid (H 3 PO 3 ), pk a1 = 2.00; (c) selenous acid (H 2 SeO 3 ), pk a1 = 2.46; (d) selenic acid (H 2 SeO 4 ), pk a1 = 1.92 9. Write the names and formulas for the strongest and weakest conjugate bases of the following acids: (a) phosphoric acid, H 3 PO 4, pk a1 = 2.12; phosphorus acid, H 3 PO 3, pk a1 = 2.00; selenous acid, H 2 SeO 3, pk a1 = 2.46; selenic acid, H 2 SeO 4, pk a1 = 1.92. (b) What are the K b values for the two bases? (c) Which base, dissolved in water to a given concentration, would produce a solution with the highest ph? 10. Determine which acid is stronger and explain why: (a) HF or HCl; (b) HClO or HClO 2 ; (c)hbro 2 or HClO 2 ; (d) HClO 4 or H 3 PO 4 ; (e) HNO 3 or HNO 2 ; (f) H 2 CO 3 or H 2 GeO 3. 11. The values of K a for phenol and 2,4,6-trichlorophenol (see structures) are 1.3 10 10 and 1.0 10 6, respectively. Which is the stronger acid? Account for the differences in acid strength. 12. Determine the concentration of H 3 O and OH in (a) 0.20 mol L 1 C 6 H 5 COOH; (b) 0.20 mol L 1 NH 2 NH 2 (hydrazine); (c) 0.20 mol L 1 (CH 3 ) 3 N (trimethylamine). 13. Calculate the ph and poh of the following aqueous solutions: (a) 0.15 mol L 1 CH 3 COOH(aq); (b) 0.15 mol L 1 CCl 3 COOH (aq); (c) 0.15 mol L 1 HCOOH(aq). 14. When the ph of a 0.10 mol L 1 HClO 2 aqueous solution was measured, it was found to be 1.2. What are the values of K a and pk a of chlorous acid? (b) The ph of a 0.10 mol L 1 propylamine, C 3 H 7 NH 2, aqueous solution was measured as 11.86. What are the values of K b and pk b of propylamine? 15. Find the nominal concentrations of the weak acid or base in each of the following aqueous solutions: (a) HClO, ph = 4.6; (b) hydrazine, NH 2 NH 2, ph = 10.2 16. The percentage ionization of benzoic acid in a 0.110 mol L 1 solution is 2.4%. What is the ph of the solution

and the pk a of benzoic acid? 17. Write the stepwise proton transfer equilibria for the ionization of: (a) sulfuric acid; (b) arsenic acid; (c) phthalic acid 18. Calculate the ph and the poh of the following solutions: (a) 0.012 mol L 1 barium hydroxide; (b) 0.026 mol L 1 nitric acid; (c) 1.4 10 4 mol L 1 hydrobromic acid; (d) 3.19 10 3 mol L 1 potassium hydroxide 19. Determine whether aqueous solutions of the following salts have a ph equal to, greater than, or less than 7. If ph > 7 or ph < 7, write a chemical equation to justify your answer. (a) NH 4 Br; (b) Na 2 CO 3 ; (c) KF; (d) AlCl 3 ; (e) KBr; (f) Cu(NO 3 ) 2 20. Determine the value of the acidity or basicity constants of the following ions: (a) NH 4 ; (b) CO 3 (c) F (d) ClO ; (e) HCO 3 ; (f) (CH 3 ) 3 NH 21. Calculate the ph of the following solutions: (a) 0.20 mol L 1 NaCH 3 CO 2 ; (b) 0.10 mol L 1 NH 4 Cl; (c) 0.10 mol L 1 AlCl 3 ; (d) 0.15 mol L 1 KCN 22. Explain what happens to (a) the concentration of hydronium ions in acetic acid solution when solid sodium acetate is added; (b) the percentage deprotonation of benzoic acid in a benzoic acid solution when hydrochloric acid is added; (c) the ph of the solution when solid ammonium chloride is added to an ammonia solution 23. A solution of equal concentrations of sodium lactate and lactic acid was found to have a ph = 3.08. (a) what are the values of K a and pk a for lactic acid? (b) what would be the ph if the acid were twice the concentration of the salt? 24. Calculate the concentration of hydronium ions in: (a) a solution that is 0.20 mol L 1 HBrO and 0.10 mol L 1 KBrO; (b) a solution that is 0.010 mol L 1 (CH 3 ) 2 NH and 0.150 mol L 1 (CH 3 ) 2 NH 2 Cl; (c) a solution that is 0.10 mol L 1 HBrO and 0.20 mol L 1 KBrO; (d) a solution that is 0.020 mol L 1 (CH 3 ) 2 NH and 0.030 mol L 1 (CH 3 ) 2 NH 2 Cl 25. The ph of 0.40 mol L 1 HF is 1.93. Calculate the change in ph when 1.0 g of sodium fluoride is added to 25.0 ml of the solution. Ignore any volume change. 26. Which indicators would you use for a titration of 0.20 mol L 1 acetic acid with 0.20 mol L 1 NaOH: (a) methyl orange (ph range 3.2-4.4); (b) litmus (ph range 5.0-8.0); (c) thymol blue (ph range 8.0-9.6); (d) phenolphthalein (ph range 8.2-10.0) Explain your selections. 27. Identify which of the following mixed systems can act as a buffered solution and write an equilibrium equation for each buffer system: (a) equal volumes of 0.10 mol L 1 HCl and 0.10 mol L 1 NaCl; (b) a solution that is 0.10 mol L 1 HClO and 0.10 mol L 1 NaClO; (c) a solution that is 0.10 mol L 1 (CH 3 ) 3 N and 0.10 mol L 1 (CH 3 ) 3 NHCl; (d) equal volumes of 0.20 mol L 1 acetic acid and 0.10 mol L 1 NaOH; (e) equal volumes of 0.20 mol L 1 HNO 3 and 0.20 mol L 1 NaOH 28. A 100. ml buffer is 0.10 mol L 1 acetic acid and 0.10 mol L 1 sodium acetate. (a) what is the ph of the buffer solution?; (b) what are the ph and the ph change resulting from the addition of 3.0 mmol of NaOH to the buffer solution?; (c) what are the ph and ph change resulting from the addition of 6.0 mmol of nitric acid to the initial buffer solution? 29. Determine K sp for the following sparingly soluble compounds given their molar solubilities: (a) AgBr, 8.8 10 7 mol L 1 ; (b) PbCrO 4, 1.3 10 7 mol L 1 ; (c) Ba(OH) 2, 0.11 mol L 1 ; (d) MgF 2, 1.2 10 3 mol L 1 30. Look up the K sp values to determine the molar solubility of: (a) Ag 2 S; (b) CuS; (c) CaCO 3 31. The molarity of CrO 4 in a saturated Tl 2 CrO 4 solution is 6.3 10 5 mol L 1, what is the K sp of Tl 2 CrO 4?

32. Calculate the solubility of each sparingly soluble compound in its respective solution: (a) silver chloride in a 0.10 mol L 1 NaCl solution; (b) mercury(ii) chloride in a 0.10 mol L 1 NaCl solution; (c) lead(ii) chloride in a 0.10 mol L 1 CaCl 2 solution; (d) iron(ii) hydroxide in a 1.0 10 4 mol L 1 FeCl 2 solution 33. Decide whether a precipitate will form when the following solutions are mixed: (a) 27.0 ml of 0.0010 mol L 1 sodium chloride and 73.0 ml of 0.0040 mol L 1 silver nitrate; (b) 1.0 ml of 1.0 mol L 1 potassium sulfate and 10.0 ml of 0.0030 mol L 1 calcium chloride and 100. ml of water 34. The concentration of magnesium, calcium, and nickel(ii) ions in an aqueous solution are 0.0010 M. (a) In what order to they precipitate when potassium hydroxide is added?; (b) determine the ph at which each salt will precipitate. 35. Use a table of K sp values to determine the solubility of each sparingly soluble substance in its respective solution: (a) aluminum hydroxide at ph = 7.0 and ph = 4.5; (b) zinc hydroxide at ph = 7.0 and ph = 6.0 36. Distinguish between the end point and the stoichiometric point of an acid-base reaction. 37. Fluoridation of city water supplies produces a fluoride ion concentration close to 5 10 5 mol L 1. Will calcium fluoride precipitate in hard water in which the calcium ion concentration is 2 10 4 mol L 1? Answers 1. a) amphiprotic; b) base; c) base; d) acid 2. a) H 2 SO 4 (aq) H 2 O(l) H 3 O (aq) HSO 4 (aq); H 2 SO 4, acid; H 2 O, base; H 3 O, conjugate acid; HSO 4, conjugate base; also possible: HSO 4 (aq) H 2 O(l) H 3 O 2 (aq) SO 4 (aq); HSO 4, acid; H 2 O, base; H 3 O, conjugate acid; 2 SO 4, conjugate base b) C 6 H 5 NH 3 (aq) H 2 O(l) H 3 O (aq) C 6 H 5 NH 2 (aq); C 6 H 5 NH 3, acid; H 2 O, base; H 3 O, conjugate acid; C 6 H 5 NH 2, conjugate base c) H 2 PO 4 H 2 O(l) H 3 O (aq) HPO 4 (aq); H 2 PO 4, acid; H 2 O, base; H 3 O, conjugate acid; HPO 4, conjugate base d) HCOOH H 2 O(l) H 3 O (aq) HCOO (aq); HCOOH, acid; H 2 O, base; H 3 O, conjugate acid; HCOO, conjugate base e) NH 2 NH 3 (aq) H 2 O(l) H 3 O (aq) NH 2 NH 2 (aq); NH 2 NH 3, acid; H 2 O, base; H 3 O, conjugate acid; NH 2 NH 2, conjugate base 3. HNO 3 = Bronsted acid; HPO 4 = Bronsted base; NO 3 = conjugate base; H 2 PO 4 = conjugate acid 4. a) [H 3 O = [OH = 1.6 10 7 mol/l; ph = 6.80; b) see (a) 5. [HCl = 0.96 mol L 1 ; [H 3 O = 0.96 mol L 1 ; [Cl = 0.96 mol L 1 ; [OH = 1.0 10 14 mol L 1 ; ph = -0.010 6. nominal molarity of Ba(OH) 2 = 0.029 mol L 1 ; [Ba 2 = 0.029 mol L 1 Ba(OH) 2 ; [OH = 0.058 mol L 1 ; [H 3 O = 1.7 10 13 mol L 1 7. a) orange juice = 5 10 4 mol L 1 ; b) saliva = 2 10 7 mol L 1 ; c) beer = 4 10 5 mol L 1 ; d) coffee = 5 10 6 mol L 1 ; saliva < coffee < beer < orange juice 8. a) 0.0076; b) 0.010; c) 0.0035; d) 0.012 9. a) strongest base is the conjugate base of weakest acid, selenous acid: H 2 SeO 3 H 2 O(l) H 3 O (aq) HSeO 3 (aq), the strongest conjugate base is HSeO 3, the hydrogen selenite ion; the weakest conjugate base is the conjugate base of the strongest acid, selenic acid: H 2 SeO 4 H 2 O(l) H 3 O (aq) HSeO 4 (aq), the weakest conjugate base is HSeO 4, the hydrogen selenate ion. b) hydrogen selenite ion: K b = 2.9 10 12 ; hydrogen selenate ion: K b = 8.3 10 13 c) HSeO 3 10. a) HCl weaker bond strength (bond strength is the major effect in the case of binary acids of elements in a given group; b) HClO 2 the more oxygen atoms attached to a given central atom in an oxoacid, the stronger the acid as the more polar the O-H bond; c) HClO 2 both have the same number of oxygen atoms, but the central atom Cl is more electronegative than Br, thus the O-H bond is more polar; d) HClO 4 both are oxoacids with 4 oxygens, but Cl is more electronegative than P; e) HNO 3 both are oxoacids of N, but HNO 3 has more oxygen atoms, thus making the O-H bond more polar; H 2 CO 3 both are oxoacids of 3 oxygens, but C is more electronegative than Ge. 11. 2,4,6-trichlorophenol is the stronger (its K a value is larger). The additional chlorines withdraw electrons from the O-H bond, thus increasing the bond polarity and making it easier to donate H. 12. a) [H 3 O = 3.6 10 3 mol L 1 ; [OH = 2.8 10 12 mol L 1 ; b) [OH = 5.8 10 4 mol L 1 ; [H 3 O = 1.7 10 11 mol L 1 ; c) [OH = 3.6 10 3 mol L 1 ; [H 3 O = 2.8 10 12 mol L 1

13. a) ph = 2.78; b) poh = 11.22; b) ph = 0.96; poh = 13.04; c) ph = 2.28; poh = 11.72 14. a) K a = 0.11; pk a = 1.0; b) K b = 5.7 10 4 ; pk b = 3.2 15. a) 0.02 mol L 1 HClO; b) 0.01 mol L 1 NH 2 NH 2 16. K a = 6.5 10 5 ; ph = 2.58 17. a) H 2 SO 4 (aq) H 2 O(l) H 3 O (aq) HSO 4 (aq); K a1 = [ H 3O [HSO 4 [H 2 SO 4 HSO 4 (aq) H 2 O(l) H 3 O 2 (aq) SO 4 (aq); K a2 = [ H 3O [SO 2-4 [HSO - 4 b) H 3 AsO 4 (aq) H 2 O(l) H 3 O (aq) H 2 AsO 4 (aq) K a1 = [ H 3O [H 2 AsO - 4 [ H 3 AsO - 4 H 2 AsO 4 (aq) H 2 O(l) H 3 O (aq) HAsO 4 (aq) K a 2 = [ H 3O [ HAsO 2-4 [H 2 AsO - 4 HAsO 4 (aq) H 2 O(l) H 3 O 3 (aq) AsO 4 (aq) K a3 = [ H 3O [ AsO 4 [HAsO 2-4 c) C 6 H 4 (COOH) 2 (aq) H 2 O(l) H 3 O (aq) C 6 H 4 (COOH)(COO )(aq) K a 1 = [ H 3O [C 6 H 4 (COOH )(COO - ) [C 6 H 4 (COOH ) 2 C 6 H 4 (COOH)(COO )(aq) H 2 O(l) H 3 O (aq) C 6 H 4 (COO ) 2 (aq) K a 2 = [H 3 O [C 6 H 4 (COO - ) 2 [C 6 H 4 (COOH)(COO - ) 18. a) ph = 12.38; poh = 1.62; b) ph = 1.59; poh = 12.41; c) ph = 3.83; poh = 10.17; d) ph = 11.50; poh = 2.50 19. a) acidic solution, ph < 7; NH 4 (aq) H 2 O(l) NH 3 (aq) H 3 O (aq); b) basic solution; ph > 7; CO 3 (aq) H 2 O(l) HCO 3 (aq) OH (aq); c) basic solution; ph > 7; F (aq) H 2 O(l) HF( aq) OH (aq); d) ph = 7; e) ph < 3 7; Al(H 2 O) 6 (aq) H 2 O(l) Al(H 2O) 5 OH 2 (aq) H 3 O 2 (aq); ph < 7; Cu(H 2 O) 4 (aq) H 2 O(l) Cu(H 2O) 3 OH (aq) H 3 O (aq) 20. (a) NH 4 (aq) H 2 O(l) NH 3 (aq) H 3 O (aq); K anh 4 = K w K bnh 3 1.8 10 5 =6.6 10 10 = [ NH 3[ H 3 O [NH 4 (b) CO 3 ; CO 3 (aq) H 2 O(l) HCO 3 (aq) OH (aq); K bco3 2 =1.8 10 4 11 K 2- ahco 3 5.6 10 (c) F, F (aq) H 2 O(l) HF( aq) OH (aq); K bf K ahf 3.5 10 4 =2.9 10 11 = [ HF[OH - [F - (d) ClO ; K bclo K ahclo 3.0 10 8 =3.3 10 7 = [ HClO[OH - [ClO - (e) HCO 3 ; K b1 HCO 3 K a1 H 2 CO 3 4.3 10 7 =2.3 10 8 = [H 2 CO 3 [OH - [HCO - 3 (f) (CH 3 ) 3 NH ; (CH 3 ) 3 NH (aq) H 2 O(l) (CH 3 ) 3 N(aq) H 3 O (aq) K a1 (CH 3 ) 3 NH = K w K b1 (CH 3 ) 3 N =1.5 10 10 5 6.5 10 21. (a) ph = 9.02; (b) ph = 5.13; (c) ph = 2.92; (d) ph = 11.24 22. (a) [H 3 O decreases; (b) % deprotonation of benzoic acid decreases; (c) ph goes down 23. (a) pk a = 3.08; (b) pk a = 2.78; 24. (a) [H 3 O = 4.0 10 9 mol L 1 ; (b) [H 3 O = 2.8 10 10 mol L 1 ; (c) [H 3 O = 1.0 10 9 mol L 1 ; (d) [H 3 O = 2.8 10 11 mol L 1 25. ph changes from 1.93 to 3.83, a change of 1.90 26. thymol blue of phenolphalein because the ph would be expected to be above 7 at the equivalence point. 27. (a) no; (b) yes; (c) yes; (d) yes; (e) no 3- -

28. (a) ph = 4.74; (b) ph = 5.0, a change of 0.27 ph units; (c) ph = 4.1, a change of 0.6 ph units 29. (a) K sp = 7.7 10 13 ; (b) K sp = 1.7 10 14 ; (c) K sp = 5.3 10 3 ; (d) K sp = 6.9 10 9 30. (a) 1.2 10 17 mol L 1 ; (b) 4.5 10 24 mol L 1 ; (c) 9.3 10 5 mol L 1 31. K sp = 1.0 10 12 32. (a) 8.0 10 10 mol L 1 ; (b) 1.3 10 16 mol L 1 ; (c) 4.0 10 4 mol L 1 ; (d) 6.3 10 6 mol L 1 33. (a) AgCl will form; (b) no precipitate will formulas 34. (a) Ni(OH) 2 first, then Mg(OH) 2, Ca(OH) 2 last; (b) Ni(OH) 2 at ph = 6.91; Mg(OH) 2 at ph = 10.14; Ca(OH) 2 when ph = 12.85 35. (a) 1.0 10 12 mol L 1 ; (b) 3.2 10 5 mol L 1 ; (c) 2.0 10 3 mol L 1 ; (d) 0.20 mol L 1 36. end point = range over which the indicator changes color; stoichiometric point point at which exactly right amount of acid and base are present to react according to the balanced chemical equation. 37. Precipitation will not occur.