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Salts, Acid-Base Relationships

conjugate acid-base pairs - HX + H2O >> X- + H3O+  

  • HX is acid
    • conjugate acid of X- (add proton)
  • H2O acts as base
    • conjugate base of H3O+ (subtract proton)
  • X- is base
    • conjugate base of HX (subtract proton)
  • H3O+ acts as acid
    • conjugate acid of H2O (add proton)
  • conjugate acid of strong base = weak acid
  • conjugate acid of weak base = strong acid
  • conjugate base of strong acid = weak base
  • conjugate base of weak acid = strong base
  • Ka x Kb = Kw
    • Kw = 1.0 x 10-14 (water autoionizes)
    • Ka = Kb >> neutral solution

Find the pH for 0.25 M of sodium lactate  

  • conjugate acid is lactic acid
  • Ka of lactic acid = 1.4 x 10-4
  • Kb = Kw / Ka = 10-14 / (1.4 x 10-4) = 7.14 x 10-11
  • Kb = [OH-] [lactic acid] / [lactate]
  • [OH-] = [(7.14 x 10-11) (0.25)]1/2 = 4.2 x 10-6
  • pOH = -log [OH-] = 5.37
  • pH = 14 - 5.37 = 8.63

hydrolysis - ability of ions to react w/ water to make H+, OH- ions  

  • anions act as bases (adding proton >> acid)
    • identify conjugate acid, use Kw to find the strength
    • anions of strong acids have negligible effect
    • anions of weak acids increase pH
  • cations act as acids (subtract proton >> base)
    • identify conjugate base, use Kw to find the strength
    • cations of strong bases have negligible effect
    • cations of weak bases decrease pH
Subject: 
Chemistry
Subject X2: 
Chemistry

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