Buffers

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Chapter 17

• Buffers

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Buffered solutions

• A solution that resists a change in pH.
• Buffers are
• A solution that contains a weak acid-weak base
  conjugate pair.
• Often prepared by mixing a weak acid, or a weak
  base, with a salt of that acid or base.

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Buffered solutions

• The solution has the ability to resist changes in
  pH upon the addition of small amounts of either
  acid or base.
• We can make a buffer of any pH by varying the
  concentrations of these solutions.

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What is the pH of a buffer that is 0.12M in
lactic acid, HC3H5O3 , and 0.10M in sodium
lactate? For lactic acid, K 1.4 x 10 -4
HC3H5O3 ? H
C3H5O3-1
I 0.12 M 0 0.10 M
C -x x x
E 0.12 x x 0.10 x
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• H 1 C3H5O3-1
• Ka HC3H5O3
• 1.4 x 10-4 (x) (0.10 x)
• (0.12 x)
• Because of the small Ka and the presence of the
  common ion, it is expected that x will be small
  relative to 0.12 or 0.10M.
• x 1.7 x 10 -4 M therefore H1 1.7 x 10
  -4 M
• pH -log (1.7 x 10 -4 M) 3.77

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Use Henderson-Hasselbach Instead

• base
• pH pKa log acid
• 0.10
• pH -log (1.4 x 10-4) log 0.12
• pH 3.85 (-0.08) 3.77

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Practice Problem

• Calculate the pH of a buffer composed of 0.12M
  benzoic acid (HC7H5O2)and 0.20M sodium benzoate.
  Ka 6.3 x 10-5
• ans 4.42
• Remember the Aqueous Equilibrium Constants are
  located in Appendix D of your textbook.

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Buffered Solutions

• Buffers resist changes in pH because they contain
  both an acidic species to neutralize OH-1 ions
  and a basic one to neutralize H1 ions.
• It is important that the acidic and basic species
  of the buffer do not consume each other through a
  neutralization reaction.

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Buffered Solutions

• HA H A-
• Ka H A- HA
• Buffers most effectively resist a change in pH in
  either direction when the concentrations of HA
  and A- are about the same. When HA equals A-
  then H equals Ka.
• Scientists usually try to select a buffer whose
  acid form has a pKa close to the desired pH.

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General equation

• Ka H A- HA
• so H Ka HA A-
• The H depends on the ratio HA/A-
• Take the negative log of both sides
• pH -log(Ka HA/A-)
• pH -log(Ka)-log(HA/A-)
• pH pKa log(A-/HA)

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This is called the Henderson-Hasselbach equation

• pH pKa log(A-/HA)
• pH pKa log(base/acid)

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Try an Acid

• Calculate the pH of the following mixture
• Prob. 1
• 0.75 M lactic acid (HC3H5O3) and 0.25 M sodium
  lactate (Ka 1.4 x 10-4)
• Answer
• pH -log(1.4x10-4) log0.25/0.75
• pH 3.38

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Now Try a Base

• Calculate the pH of the following mixture
• 0.25 M NH3 and 0.40 M NH4Cl
• (Kb 1.8 x 10-5)
• Answer
• NH3 NH41 OH-1
• (NH3 is B, NH41 is HB1)
• pOH pKb log HB1 / B
• pOH -log(1.8x10-5) log0.4/0.25
• pOH 4.94 now convert to pH

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Buffering Capacity

• This is the amount of acid or base the buffer can
  neutralize before the pH begins to change to an
  appreciable degree.

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Buffering Capacity

• The buffering capacity, that is, the
  effectiveness of the buffer solution, depends on
  the amount of acid and conjugate base from which
  the buffer is made.
• The larger the amount the greater the buffering
  capacity.
• In general, a buffer system can be represented as
  salt-acid or conjugate base-acid.

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Buffer capacity

• The pH of a buffered solution is determined by
  the ratio A-/HA.
• As long as this doesnt change much the pH wont
  change much.
• The more concentrated these two are the more H
  and OH- the solution will be able to absorb.
• Larger concentrations means bigger buffer
  capacity.

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Buffer Capacity

• Calculate the change in pH that occurs when 0.010
  mol of HCl(g) is added to 1.0L of each of the
  following
• 5.00 M HAc and 5.00 M NaAc
• 0.050 M HAc and 0.050 M NaAc
• Ka 1.8x10-5

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Buffer capacity

• The best buffers have a ratio A-/HA 1
• This is most resistant to change
• True when A- HA
• Make pH pKa (since log10)

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Addition of Strong Acids or Bases to Buffers

• See pg. 648 BL for explanation and examples.

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Adding a strong acid or base

• Do the stoichiometry first.
• A strong base will take protons from the weak
  acid reducing HA0
• A strong acid will add its proton to the anion of
  the salt reducing A-0
• Then do the equilibrium problem.

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B L pg 649
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• Try the Buffer worksheet from the Chang
  chemistry text.

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Prove theyre buffers

• What would the pH be if 0.020 mol of HCl is added
  to 1.0 L of both of the preceding solutions.
• What would the pH be if 0.050 mol of solid NaOH
  is added to each of the proceeding.

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• Thanks to Mr. Green