Solutions for Acid-Base Equilibria and Titrations Exercises

Solutions for Acid-Base Equilibria & Titrations Exercises 1. HA-(aq) + H2O(l) º H3O+(aq) + A2-(aq) Ka = 10-8 HA-(aq) + H2O(l) º H2A(aq) + OH-(aq) Kb = 10-11 Since Ka>>Kb, the acid dissociation reaction is favored. However, the H3O+ produced in this reaction will react with the OH- formed in the base hydrolysis and drive this reaction further to the right than might be expected from the value of Kb. 2. a) H2M(aq) + H2O(l) init 0.100 M equil 0.100 - x º H3O+(aq) + HM-(aq) ~10-7 M 0 ~x x Y x = 0.0119 M (x << 0.1 assumption is not valid! - use successive approximations) x2 = 0.0112 x3 = 0.0112 M = [H3O+] = [HM-] Y pH = 1.95 and [H2M] = 0.100 M - 0.0112 M = 0.089 M Since Ka2 << Ka1, the second acid dissociation should not change these concentrations much. HM-(aq) + H2O(l) init 0.0112 M equil ~0.0112 M º H3O+(aq) + M2-(aq) 0.0112 M 0 ~0.0112 M x Solutions for Acid-Base Equilibria & Titrations Exercises b) 0.100 M NaHM (amphiprotic system) HM-(aq) + H2O(l) º H3O+(aq) + M2-(aq) HM-(aq) + H2O(l) º H2M(aq) + OH-(aq) For this amphiprotic substance HM- (assuming [HM-] = 0.100 M) [H3O+] = 5.3 x 10-5 M Y pH = 4.28 Solutions for Acid-Base Equilibria & Titrations Exercises c) 0.100 M Na2M M2-(aq) + H2O(l) HM-(aq) + H2O(l) º HM-(aq) + OH-(aq) º H2M(aq) + OH-(aq) Since Kb2 << Kb1, treat salt as monobasic M2-(aq) + H2O(l) º HM-(aq) + OH-(aq) 0.100 M 0 ~10-7 M equil 0.100 - x x ~x init Y x = 2.23 x 10-5 M = [HM-] = [OH-] Y pOH = 4.65 and pH = 9.35 also, [M2-] = 0.100 M Solutions for Acid-Base Equilibria & Titrations Exercises 3. For acid HA, pKa = 7.00 (Ka = 1.0 x 10-7) When pH = pKa, [A-] = [HA] If pH < pKa, [HA] > [A-] If pH > pKa, [A-] > [HA] 4. a) at pH = 6.00, [H3O+] = 1.0 x 10-6 Y [HA] > [A-] b) at pH = 8.00, [H3O+] = 1.0 x 10-8 Y [A-] > [HA] c) at pH = 6.40, [H3O+] = 3.98 x 10-7 Y [A-] / [HA] = 0.25 For the HCO3-/CO32- buffer, if pH = 10.00 Y [H3O+] = 1.0 x 10-10 M and 5.00 g NaHCO3/100 mL Y [HCO3-] = 0.5952 M HCO3-(aq) + H2O(l) init 0.5952 M equil ~0.5952 M º H3O+(aq) + CO32-(aq) ~10-7 M x 1.0 x 10-10 M ~x