Additional Aspects of Aqueous Equilibria

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Additional Aspects of Aqueous Equilibria
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Buffer Solutions

• solution that resists changes in pH upon addition
  of small amounts of strong acid or base
• consists of a mixture of a weak acid and its
  conjugate base
• it works b/c the weak acid can neutralize small
  amounts of strong base and the weak base can
  neutralize small amounts of strong acid

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Common Ion Effect

• decreases the ionization of a weak electrolyte
  when a common ion is added to the solution
• Consider this
• CH3COOH H2O CH3COO- H3O
• adding NaCH3COO to the solution will increase
  CH3COO-, causing the rxn to shift left, thus
  decreasing the ionization of acetic acid
• resulting solution is a buffer containing a weak
  acid a conjugate weak base

4
To make a buffer solution, what weak acid must be
added to an amount of sodium phosphate? Write a
chemical reaction and its corresponding
equilibrium expression.
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Understanding A/B reactions

• SA SB ? NS H2O
• HCl NaOH ? H2O NaCl
• H OH- ? H2O
• soln is neutral b/c both the acid and base ions
  are neutral ions
• SA WB ? WA NC
• HCl NaCH3COO ? NaCl CH3COOH
• H CH3COO- ? CH3COOH
• soln is slightly acidic
• SB WA ? WB NC
• NaOH CH3COOH ? NaCH3COO H2O
• OH- CH3COOH ? CH3COO- H2O
• soln is slightly basic
• WA WB ? does not measurably react

6
Write and balance complete and net ionic
equations for the reaction of ammonia solution
with nitric acid.
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Calculating Buffer pH

• calculated the same as the pH of a weak acid
  soln - RICE
• What is the pH of an aqueous mixture containing
  0.20 M acetic acid and 0.10 M sodium acetate?

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If the soln contains the base is in excess, the resulting soln is a the acid is in excess, the resulting soln is a neither acid nor base is in excess, the resulting soln is a
SA SB SB SA neutral
SA WB buffer SA WA
WA SB SB buffer WB
WA WB buffer buffer buffer
9
Explain how nitric acid and sodium acetate can be
used to make a buffer solution. Illustrate your
answer using a chemical equation.
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What is the pH of a solution made by mixing 1.0 L
of 0.10 M HCl with 2.0 L of 0.060 M NaOH
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What is the pH is a soln made by mixing 0.11 M
HCl with 3.0 L of 0.080 M NaF?
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What is the pH of a soln made by mixing 2.5 L of
0.20 M CH3COOH with 1.0 L of 0.30 M KCH3COO?