Equilibria of Weak Acids

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Equilibria ofWeak Acids BasesPart II Ka and
Kb

• Dr. C. Yau
• Fall 2009

Based on Brady Senese 5/eChap. 16 Sec 2
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Relationship between Ka and Kb
Review Ka x Kb Kw 1.0 x 10-14
pKa pKb
14.00 Calculations can be (1) to determine
Ka and Kb (2) to make use of Ka and Kb
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Calculations to find Ka and Kb
Pract Exer 7 p.648 The methylammonium ion CH3NH3
has a Ka of 2.3x10-11. What is the Kb for the
base methylamine? Ka x Kb 1.0 x 10-14
Do Pract Exer 8 p.648
Review Write the equilibrium expression for Ka
and Kb in this problem.
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Ka x Kb 1.0x10-14

• If Ka is large, Kb must be small.
• If Ka is small, Kb must be large.
• Ka and Kb are INVERSELY PROPORTIONAL.
• When comparing 2 acids,
• the weaker acid produces
• the stronger conjugate base.

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Stronger acids give weaker conjugate bases.
CONJUGATE BASES
ACIDS
Fig. 16.1 p.649
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• Example 16.5 p.650
• Lactic acid (HC3H5O3), which is present in sour
  milk, also gives sauerkraut its tartness. It is
  a monoprotic acid.
• In a 0.100 M soln of lactic acid, the pH is 2.44
  at 25oC. Calc the Ka and pKa for lactic acid at
  that temperature.

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• Example 16.6 p.651
• Methylamine, CH3NH2, is a weak base and one of
  several substances that give herring brine its
  pungent odor.
• In 0.100 M CH3NH2 only 6.4 of the base is
  ionized. what are Kb and pKb of methylamine?
• What does "6.5 ionized" mean?

Do Pract Exer 9, 10, 11 p.652
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Determining the pH Of Aqueous Weak Acid Solutions

• Dominant equilibrium is Ka reaction
• write the net ionic equation
• look up the Ka value for the acid
• set up ICE table
• solve for x
• Calculate pH from the hydronium concentration at
  equilibrium

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• Example 16.7 p.654
• A student planned an experiment that would use
  0.10 M propionic acid, HC3H5O2.
• Calculate the value of H and the pH for this
  soln.
• For propionic acid, Ka 1.34x10-5
• Do Pract Exer 12, 13 p.655

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• Example 16.8 p. 655
• A solution of hydrazine, N2H4 has a conc of 0.25
  M. what is the pH of the soln, and what is the
  percentage ionization of the hydrazine? Hydrazine
  has Kb 9.6x10-7
• Do Pract Exer 14, 15, 16 p.655

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• SALT SOLUTIONS
• HCl NaOH H2O NaCl
• We say that HCl has been neutralized by NaOH.
  The product mixture is neutral.
• HCl NH4OH H2O NH4Cl
• The product mixture is acidic! Why?
• NH4Cl dissociates into NH4 and Cl-
• and NH4 is acidic!
• NH4 H2O NH3 H3O

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• In the same way,
• HC2H3O2 NaOH H2O NaC2H3O2
• The product mixture is not neutral.
• NaC2H3O2 dissociates into Na and C2H3O2- and
  acetate is the conjugate base of a weak acid
  (acetic acid).
• C2H3O2- is a strong base. It will pull a proton
  off water
• C2H3O2- HOH C2H3O2H OH-
• producing OH- ions.
• THUS SODIUM ACETATE PRODUCES A BASIC SOLUTION!

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Predicting AcidBase Properties Of A Salt

• 1. If neither the cation nor the anion can affect
  the pH, the solution should be neutral
• 2. If only the cation of the salt is acidic, the
  solution will be acidic
• 3. If only the anion of the salt is basic, the
  solution will be basic
• 4. If a salt has a cation that is acidic and an
  anion that is basic, the pH of the solution is
  determined by the relative strengths of the acid
  and base
• compare Ka and Kb for the ions

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Predicting Effects Of Ions On pH

• Most metal cations are neutral or extremely weak
  acids
• Conjugate acids of weak bases are acidic
• Conjugate bases of weak acids are basic
• Conjugate bases of the strong acids are neutral
  (except HSO4- which is still fairly acidic)

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• 0.1M solutions of the following are acid/ base/
  neutral or amphoteric?
• HCl
• NaCl
• NaCN
• HCN
• Na2S
• Na3PO4
• NH4Cl

• Acid
• Neutral
• Base
• Acid
• Base
• Base
• acid

Do Example 16.9, Pract Exer 17 18.
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• At this point, you are expected to recognize
  whether a salt will produce an acidic or basic
  solution from looking at its formula (see last
  slide, and Example 16.9, Pract Exer 17 18).

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