Ch' 15: Applications of Aqueous Equilibria

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Ch. 15 Applications of Aqueous Equilibria
15.2 Buffers
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Buffered Solutions

• a solution that resists pH change when H or OH-
  is added
• made from
• weak acid and its salt
• weak base and its salt

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Example 4

• A buffered solution contains 0.5 M HC2H3O2
  (Ka1.8x10-5) and NaC2H3O2. Find pH of the
  solution
• major species HC2H3O2, Na, C2H3O2-, H2O

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Example 4

• What would happen to the pH if we added 0.010
  mol of solid NaOH was added to 1.0 L of this
  buffered solution?

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Example 4

• major species HC2H3O2, Na, OH-, C2H3O2-, H2O
• HC2H3O2 OH- ? C2H3O2- H2O
• goes to completion b/c of SB

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Example 4

• Now that we know the amount of acetic acid left,
  we must find out how much H is created from it.

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How does it do that?

• if it is a WA and salt
• it contains large amounts of weak acid, HA, and
  its conjugate base, A-.
• when OH- is added, the HA donates H to it so the
  OH- does not buildup
• when H is added, A- accepts the H so H does
  not buildup either
• as long as the change is small compared to the
  original conc, pH wont change much

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How does it do that?

• if it is a WB and salt
• it contains large amounts of weak base, B, and
  its conjugate base, BH.
• when OH- is added, the BH donates H to it so
  the OH- does not buildup
• when H is added, B accepts the H so H does not
  buildup either
• as long as the change is small compared to the
  original conc, pH wont change much

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How does it do that?
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Henderson-Hasselbalch equation

• useful for calculating the pH of a solution when
  the concentrations of acid and base are known and
  the change (x) is negligible
• for a certain acid-base pair, if they have the
  same ratio, they have the same pH

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Example 5

• Find the pH of 0.75 M HC3H5O3 (Ka1.4x10-4) and
  0.25 M NaC3H5O3
• major species HC3H5O3, Na, C3H5O3-, H2O

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Ch. 15 Applications of Aqueous Equilibria

• 15.3 Buffer Capacity

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Buffer Capacity

• amount of H or OH- the solution can absorb
  without a significant change in pH
• determined by the size of HA and A-
• if the amounts are equal (ratio of one) and they
  have large concentrations, it will have the
  largest buffer capacity
• the pKa of weak acid used should be very close to
  the pH desired for the buffer

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Buffer Capacity

• because the HA is so low in solution B, the
  buffer capacity is much lower
• this causes the change in pH after adding acid to
  be much larger

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Example

• A buffered solution contains 0.25 M NH3
  (Kb1.8x10-5) and 0.40 M NH4Cl. Calculate the pH
  of this solution.

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Example
What would the pH be after 0.020 mol of OH- is
added to 500.0 mL of the solution?
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Example