Carboxylic acids are the most common class of weak acids

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Weak Acids and Bases

• Carboxylic acids are the most common class of
  weak acids
• Amines are the most common class of weak bases

Ka 1.75 x10-5

Acetic Acid (HA)
Acetate (A-)
Kb 4.4 x10-4


Methylamine (B)
Methylammonium (BH)
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Conjugate Acids and Bases

• An organo-ammonium ion is the conjugate weak acid
  of an amine (weak base)

Kb 4.4 x10-4


Methylamine (B)
Methylammonium (BH)
Ka 2.3 x10-11

Methylamine (B)
Methylammonium (BH)
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Predicting Acidity/Basicity

• If you dissolve methylammonium chloride in water
  will the solution be acidic or basic?
• We would expect the solution to be slightly
  acidic, because the ammonium ion is acidic but
  chloride is not basic.

Conjugate to a weak base (weak acid)
Conjugate to strong acid (no basicity)
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Polyprotic Acids and Bases

• Polyprotic acids and bases can donate or accept
  more than one proton.
• Equilibrium constants for polyprotic compounds
  follow the notation
• K1, K2, K3
• Kb1, Kb2
• K1 refers to the acid with the most protons and
  Kb1 to the base with the fewest protons.

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Oxalic Acid

• Oxalic acid is a diprotic carboxylic acid

K1 5.6 x10-2

Oxalic acid
Monohydrogen oxalate
K2 5.4 x10-5

Oxalate
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Phosphate

• Phosphate is a triprotic base

Kb1 1.4 x10-2


Phosphate
Monohydrogen phosphate
Kb2 1.6 x10-7


Dihydrogen phosphate
Kb3 1.4 x10-12


Phosphoric acid
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Relationship Between Ka and Kb

• Ka and Kb of a conjugate acid-base pair in
  aqueous solution are related
• KaKbKw for any conjugate acid-base pair in
  aqueous solution

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Example

• What is the Kb for acetate ion if the Ka for
  acetic acid is 1.75 x 10-5?

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Polyprotic acids

• For a diprotic acid
• For a triprotic acid