Part I: Common Ion Effect, Buffers, and Titrations

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Part I Common Ion Effect,Buffers, and
Titrations

• Thursday, July 19th
• CHM 102

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Common Ion Effect

• Dont be intimidated by wording!
• Common ion- Just as you can have something in
  common with a friend, two different substances
  can have an ion in common!
• When NaClO4 and perchloric acid (HClO4)
  dissociate in water, they share an ion in common
  (the perchlorate ion, ClO4-).

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Practice Common Ion Effect

• Calculate the pH of a solution containing 0.085 M
  nitrous acid. (Ka 4.510-4)

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Practice Common Ion Effect

• Calculate the pH of a solution containing 0.085 M
  nitrous acid (HNO2) and 0.1 M potassium nitrite
  (KNO2). (Ka 4.510-4)

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Buffers

• Buffers (or buffered solutions) are solutions
  which resist changes in pH.
• The consist of a solution containing a weak acid
  and its conjugate base.
• Note that buffers CANNOT BE composed of a strong
  acid and its conjugate base!

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Practice Buffers

• Which of the following acid/base pairs could be
  used in a buffer solution?
• H2SO4 and HSO4-
• HC2H3O2 and C2H3O2-
• H3PO4 and H2PO4
• NH4 and NH3

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Buffers

• Think of an equilibrium of a weak acid in
  solution, well use HF
• HF(aq) H2O(l) ? F-(aq) H3O(aq)
• At equilibrium, weve got both HF(aq) and F-(aq),
  which is a conjugate weak acid/weak base pair.
• The equilibrium for this reaction is shown by

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Buffers-pH calculation

• If we have HF and F- solution as a buffer and we
  drop in a small amount of dilute NaOH, what will
  happen? What will happen if we drop in dilute
  HCl?
• The idea here is that acid added to the buffer
  will react and be consumed by the conjugate base
  component (F-) of the buffer solution.
• Likewise, should base be added to the buffer
  solution, it would be consumed by the HF in
  solution.
• This is how the buffer solution resists pH change!

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Buffers-pH calculation

• If the Ka for lactic acid is 1.410-4, what is
  the pH of a solution containing 0.5 M lactic acid
  and 0.5 M sodium lactate?
• To do a problem such as this, wed use the
  Henderson-Hasselbalch equation pH
  pKa log(c.base/c.acid)
• The pKa is just the log(Ka) just as the pH
  is the log(H). pH 3.85
  log(0.5/0.5)
• pH 3.85

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Buffers-pH calculation

• What concentration of sodium benzoate must be
  added to a solution of 0.2 M benzoic acid to have
  a pH of 4.00? (Ka 6.310-5) pH
  pKa log(c.base/c.acid)

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Buffers- Capacity and pH Range

• Buffers can only neutralize so much acid or base
  until additions of either begin to significantly
  effect the pH of the buffered solution.
• The buffer capacity is the amount of acid or
  base a buffered solution can neutralize with
  little appreciable pH variation.
• The effective pH range of a buffer usually lies
  near its pKa, with equal molar concentrations of
  conjugate acid and base.
• pH pKa log(c.base/c.acid)

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Choosing a buffer

• Because pH pKa when the molar concentrations of
  conjugate base and acid are equal, the choice of
  a buffer for a desired pH range can be based upon
  known Ka values.
• Which one of the options below would I choose if
  I wanted my pH to be buffered at pH 4.7?

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Titrations

• Titrations involve slowly adding a base of known
  concentration to an acid. (or known acid to
  base)
• pH is usually recorded and plotted against the
  volume added of the known titrant.

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Titrations-Strong Acid with Strong Base
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Titration-Weak Acid with Strong Base
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Titration-Strong Acid with Weak Base
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Titrations- Polyprotic Acids
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Titration problem

• Calculate pH in the solution formed by adding 10
  mL 0.1 M HCl to 20 mL 0.1 M NH3 (Kb1.810-5).

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Titration problem

• Calculate pH at equivalence where 40.0 mL benzoic
  acid is titrated with 0.050 M NaOH.

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Titration problem

• Calculate pH at equivalence where 40.0 mL 0.1 M
  NH3 is titrated with 0.100 M HCl.

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Exam 3 Review

• Thursday, July 19th
• CHM 102