COMMON ION

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COMMON ION EFFECT
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COMMON ION

• an ion common with one in a system at equilibrium
  which places a stress on the equilibrium
• Common Ion

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Uses of Common Ion Effect

• 1. control pH of a weak acid or base
• 2. control formation of a precipitate

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BUFFER

• A solution which resists a change in pH when an
  acid or base is added
• consists of a weak acid or base and a salt
  containing a common ion of its conjugate

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How does LeChateliers Principle explain the
operation of a buffer?
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Example of a buffer system

• CH3COOH HOH ?? CH3COO- H3O
• NaCH3COO(aq) ? Na CH3COO-

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Characteristics of a Good Buffer
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• 1. operates over a narrow pH range (lt 1 pH unit)
• 2. no reactions between buffers in a multiple
  buffer system
• 3. range can be extended using more than one
  buffer

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Henderson-Hasselbalch Equation
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Maximum buffering will occur when ratio is close
to 1, or when pH pKa
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1. What is the pH of a 0.20 M acetic acid
solution?
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Add 10.0 mL of 0.20 M NaOH to 50.0 mL of the
preceding solution. What is the pH?
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Add 5.0g sodium acetate (MM 82.05) to 500. mL of
the 0.20 M acetic acid solution. What is the pH?
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Add 10.0 mL of 0.20 M NaOH to 50.0 mL of the
preceding solution. What is the pH?
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2. Calculate the mass of ammonium chloride (MM
43.6) needed to buffer 250. mL of 2.0 M ammonia
to a pH of 10.
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