Titration of a weak base with a strong acid

1
Monoprotic Acid - Base Systems

• Titration of a weak base with a strong acid
• Outline of titration of 0.1000 M NH3 with 0.1000
  M HCl
• Equilibria
• NH3 H2O NH4 OH-
• 2H2O H3O OH- Kw
  H3OOH-
• Charge balance equation
• NH4 H3O OH- Cl-
• 0.00 mL added HCl solution basic, Cl- 0, so
  NH4 ? OH-, NH3 ? CNH3
• Between 0.00 mL and equivalence point buffer
  region
• NH4 ? CNH4 NH3 ? CNH3
• Equivalence point solution of 0.05000 M NH4
  use weak acid calculation
• Beyond equivalence point solution of a strong
  acid, HCl H3O ? CH3O

2
Monoprotic Acid - Base Systems

• pH of dilute solutions of very weak acids
• Example 5 x 10-5 M phenol, fOH
• fOH H2O fO- H3O
• 2H2O H3O OH- Kw
  H3OOH-
• Making the usual assumptions,
• fO- ? H3O, fOH ? CfOH,
• A more complete analysis
• Charge balance fO- OH- H3O- ?
• Mass balance CfOH fO- fOH but fO- ltlt
  fOH
• Using Ka

3
Monoprotic Acid - Base Systems

• Acid - base indicators are weak acids or bases
  whose conjugate acid has a different color from
  the conjugate base.
• color 1 color 2
• HIn1 H2O In-1 H3O
• In-2 H2O HIn2 OH-
• Color 2 predominates if In-1 10 x HIn1 or
  HIn2 10 x In-2
• Color 1 predominates if HIn1 10 x In-1 or
  In-2 10 x HIn2
• For indicator system 1,
• color 1
• color 2
• Likewise for indicator system 2, color changes
  over pH range pKb ?1

?olor changes from color 1 to color 2 over a
pH range pKa 1
4
Monoprotic Acid - Base Systems

• Acid - base indicators
• See Table 10-1, FAC7 p. 191, for transition range
  of various acid base indicators, the pKa of the
  indicators and the color change of each
  indicator
• See inside front cover of FAC7 for a colorful
  chart of indicators
• Types of indicators
• Phthalein indicators
• phenolphthalein

5
Monoprotic Acid - Base Systems

• Acid - base indicators
• Types of indicators
• Sulfonphthalein indicators bromocresol green

6
Monoprotic Acid - Base Systems

• Acid - base indicators
• Types of indicators
• Azo indicators methyl orange (methyl red has
  -COO- in place of -SO3-)

7
Monoprotic Acid - Base Systems

• The shape of titration curves and choosing an
  indicator
• Strong acid - strong base see Fig. 10-4. FAC7 p.
  98 for indicator choices including the effect of
  reagent concentrations
• Weak acid - strong base see Figs. 10-11 and
  10-12, FAC7 p. 213 for indicator choices
  including the effects of reagent concentrations
  and Ka
• Weak base - strong acid see Fig. 10-13, FAC7 p
  216 for indicator choices including the effects
  of Kb
• Dissecting the titration curve of weak acids with
  strong base or weak base with strong acid
• Initial pH weak acid or weak base calculation
• Between initial pH and equivalence point pH of a
  solution containing a weak acid and its
  conjugate base or weak base and its conjugate
  acid
• Buffer calculation use Henderson-Hasselbach
  equation or a modification if the solution is
  dilute
• Equivalence point calculation pH of a solution
  of the conjugate base of the weak acid or
  conjugate acid of the weak base
• Beyond the equivalence point pH of a solution
  containing a strong base or strong acid

8
Monoprotic Acid - Base Systems

• Buffers are solutions that resist change in pH as
  a result of dilution of the solution or the
  addition of an acid or base to the solution
• Consider a solution prepared by mixing 50.00 mL
  Hac with 10.00 mL NaOH

?ompare this to diltuting a pH 4.16 solution of
HCl by 10,000 H CHCl 6.92 x 10-6 H ?
CHCl H 1.00 x 10-7 pH7.00