What is a buffer?

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What is a buffer?
A solution containing a conjugate acid/base pair
A solution of a weak acid and the salt of a weak
acid.
A solution of a weak base and the salt of a weak
base.
A solution that resists changes in pH upon the
addition of either strong acids or strong bases.
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Example Acetate buffer
A solution is made 0.100M in acetic acid
(CH3COOH) and 0.100M in sodium acetate
(NaCH3COO).
Sodium acetate is completely
soluble . before dissolution 0.100
M 0 M 0 M
NaCH3COO ? Na CH3COO- After dissolution 0
M 0.001 M 0.001 M
Before dissociation 0.100 M
CH3COOH H2O ? H3O CH3COO-
After dissociation (0.100 x) M
x M (0.100 x) M
Ka 1.8 x 10-5 H3OCH3COO-/CH3COOH
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The Henderson-Hasselbach Equation derivation
HA H2O ? H3O A-
Ka H3O A-/HA
pKa - log Ka - log (H3O A-/HA)
pKa - log H3O - log (A-/HA)
pH pKa log (A-/HA)
pKa pH - log (A-/HA)
pH pKa log (base/acid)
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pH pKa log (base/acid)
Try different combinations of CH3COO-/CH3COOH!
0.10 M acid and 0.10 M base 0.10 M
acid 0.02 M base 0.10 M acid and 0.20 M
base etc.
Try a different acid/conjugate base combination.
HCN/CN- Ka 4.0 x 10-10 find pKa -
insert s of acid and base into HH equation
Try a weak base/conjugate acid combination.
NH3/NH4 Kb 1.8 x 10-5 find Ka and
pKa insert s of acid and base into HH
equation
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General Buffer Concepts

• A buffer is most effective when pH pK.
• At this pH acid conjugate base.
• The buffer is equally effective against added
  acid or added base.

• The buffer loses effectiveness when added
  acid/base
• exceeds the buffer.

Some Buffers acetate acetic acid sodium
acetate pKa 4.74 phosphate H2PO4- and
HPO42- pKa2 7.21 ammonium NH4Cl
NH3 pKa 9.26
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Qualities of a good buffer solution. 1) The
solution contains conjugate weak acid/base pair
2) The pH is within 1 pH unit of the pK for the
buffer system. 3) The total buffer gt acid or
base that will be added.
Preparing a buffer solution

• Make a solution of a weak acid and the salt of
  the weak acid until
• both the pH and the total buffer is the
  desired value

2) Make a solution of a weak acid and add a
strong base until the pH is at the desired
value.
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Preparing Buffer solutions
1. By adding the conjugate acid/base combination
Make a 0.20 M ammonium buffer at pH 9.0?
pH pK log (NH3/NH4)
9.00 9.26 log (0.2 x)/x x 0.13
M NH4
NH3 0.2 x 0.07 M

• By adding the strong acid a weak base (or salt of
  weak acid)
• or adding strong base to weak acid (or salt
  of weak base)

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pH pKa log (base/acid)
Acetate buffer adding acid or base
CH3COOH H2O ? H3O CH3COO-
What happens if 0.050M HCl is added to a
solution containing 0.10 M CH3COOH and 0.10 M
NaCH3COO?
Qualitatively use LeChateliers Principle
Quantitatively Use HH equation Assume all of
acid reacts with conjugate base This will
increase acid and decrease base by amount of
HCl added
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Neutralization Reactions
Strong Acid and strong base
HCl(aq) NaOH(aq) ? H2O(l) NaCl(aq)
What happens to the pH as a strong base solution
is added to a strong acid?
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pH - log H
Begin with 0.1 L H2O - titrate with 0.1M HCl
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Begin with 0.1 L of 0.1M NaOH - titrate with 0.1M
HCl
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Begin with 0.1 L of 0.1M NH3 - titrate with 0.1M
HCl
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Begin with 0.1 L of 0.1M NaCH3COO - titrate with
0.1M HCl
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Strong acid added to strong base
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Strong acid added to weak base
or conjugate base of weak acid
13
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Buffering zone Center pKa of buffer.
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