CHAPTER SEVENTEEN ADDITIONAL ASPECTS OF AQUEOUS EQUILIBRIA Water is the most common and the most important solvent on earth. We have discussed acid-base equilibria in the previous two chapters. In this chapter will discuss equilibria involved slightly soluble salts and those involving complex ions. The Common Ion Effect What happens when NaC 2 H 3 O 2 is added to a solution of HC 2 H 3 O 2 ? NaC 2 H 3 O 2 (aq) → Na + (aq) + C 2 H 3 O 2 - ( aq) (strong electrolyte) completely ionized HC 2 H 3 O 2 (aq) H + (aq) + C 2 H 3 O 2 - (aq) Partially ionized Addition of C 2 H 3 O 2 - from NaC 2 H 3 O 2 causes the above reaction to shift to the left. The dissociation of the weak acid, HC 2 H 3 O 2 , decreases when we add the strong electrolyte, NaC 2 H 3 O 2 , which has an ion in common with it. 1
Common Ion Effect: The ionization of a weak electrolyte is decreased by adding to the solution a strong electrolyte that has an ion in common with the weak electrolyte. Example 1: Problem 17.13. Calculate the pH of a solution that is 0.060 M potassium propionate (KC 3 H 5 O 2 ) and 0.085 in propionic acid (HC 3 H 5 O 2 ). K a = 1.3 x 10 -5 . HC3H5O2 <-> H+ + C3H202- I .085 0 0 C -X X X E .085-X X X K= X 2 /.085-X = 1.3X10 -5 X 2 /.085= 1.3X10 -5 X 2 = 1.05X10 -6 X=1.05X10 -3 =H+ 1.05X10 -3 /.085=1.24% pH= 2.98 HC3H5O2 <-> H+ + C3H202- I .085 0 .060 C -X X X E .085-X X .060+X K= (X)(.06x)/ (.085-x)= 1.3x10 -6 .060x/.085=1.3x10 -6 x= 1.105x10 -6 /0.60 x=1.84x10 -5 2
1.84x10 -5 /0.085=.022% ionization has been suppressed by addition of a common ion. Example 2: Practice Exercise, p 724. Calculate the formate, CHO 2 - , ion concentration, the formic acid concentration and the pH of a solution that is 0.050 in formic acid (HCHO 2 ; K a = 1.8 x 10 -4 ) and 0.10 in HNO 3 . Buffered Solutions Solutions which contain a weak conjugate acid-base pair, can resist drastic changes in pH upon the addition of small amounts of strong acid or strong base are called buffered solutions (or buffers) . Composition and Action of Buffered Solutions A buffer resists changes in pH because it contains both acidic species to neutralize OH - ions and and basic species to neutralize H + ions A buffer is made up of a weak acid-base conjugate pair.
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