Chapter 18: Buffers and Titrations

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Chapter 18 Buffers and Titrations
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Buffers

• a system will maintain a relatively constant pH
  if it already contains a quantity of acid and a
  quantity of base.
• DEFINE A solution that is relatively resistant
  to changes in pH is a BUFFER.
• For this class that will be a solution of a weak
  acid and its conjugate base.
• HCOOH(aq) ltgt HCOO-(aq) H(aq)
• formic acid Na formate
• NOTE With buffers you are NOT starting from
  either side, but from BOTH sides

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Determination of Buffer pH

• HB B- at equilibrium ? concentrations on
  mixing
• HB
• H Ka
• B-
• What is the pH of a formic acid/Naformate buffer
  if 1.0 mole of each are dissolved in 1.0L
  solution?
• HCOOH 1.0M
• H Ka 1.8 x 10-4 1.8 x 10-4
• HCOO- 1.0M
• pH 3.74
• note if HB ???B-, then pH pKa

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Determination of Buffer pH cont.

• I want 1.0L of the above buffer to have a pH
  5.20. If I have a 0.10M formic acid solution, how
  many moles of solid Naformate should be added?
• pH 5.20 H 6.3 x 10-6
• HCOO- Ka HCOOH (1.8 x
  10-4)(0.10M)
• H
  6.3 x 10-6
• HCOO- 2.8M
• 2.8M x 1.0L 2.8 moles

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Effect of Added Strong Acid or Base on Buffers

• if add 0.1 mole H to water, pH will go from 7 to
  1 (106 difference in H).
• HCOOH(aq) ltgt HCOO-(aq) H(aq)
• what will happen if we add a strong acid to a
  HCOOH/HCOO- buffer?
• ---the strong acid will react with the weak base
• HCOO-(aq) H(aq) -----gt HCOOH(aq)
• What happens to a system when its equilibrium is
  perturbed?

100 based on this limiting reagent
will change concentrations of both of these
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Quantitative Example

• start with 1.0L buffer which is 1.0M in both acid
  and conjugate base.
• HCOOH 1.0M
• H Ka 1.8 x 10-4 1.8 x 10-4
• HCOO- 1.0M
• pH 3.74
• add 0.10 mole H what is new pH?
• HCOO-(aq) H(aq) -----gt
  HCOOH(aq)
• start 1 mole 0.10 mole 1 mole

Calculation works best with moles
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Acid-Base Titrations

• from lab
• adding base from buret until moles of base added
  moles of acid in flask (equivalence point).
• n(OH-) n(H) or MH x VH MOH- x
  VOH-
• how do we determine when we have reached the
  equivalence point?
• INDICATORS weak organic acids or bases which
  have different colors in 2 ionized states.
• HIn ltgt In- H
• e.g. phenolphthalein colorless red
• relative quantities of conjugate acid/base pair
  determines color of the solution
• since the relative quantities depend on H, the
  color will change as pH changes
• LeChateliers Principle, again.

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Choosing Indicators

• it is important to choose an indicator which will
  change color near the pH of the equivalence point
• what will the predicted pH be?
• Strong Acid / Strong Base pH 7
• Strong Acid / Weak Base must calculate
• Strong Base / Weak Acid must calculate
• be careful!
• if indicator changes color before the endpoint,
  the reaction will not be complete.
• if it changes after the endpoint, too much
  reagent will have been added.

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Strong Acid / Strong Base Titration Curve

• --start with 50mL 1.0M HCL pH 0
• --add 10.0mL 1.0M NaOH (0.67 M HCl
  left) pH 0.18
• --add 40.0mL 1.0M NaOH (0.11 M HCl
  left) pH 0.95
• --add 49.0mL 1.0M NaOH (0.010 M HCl
  left) pH 2.0
• --add 49.9mL 1.0M NaOH (0.001 M HCl
  left) pH 3.0
• --add 49.99mL 1.0M NaOH (0.0001 M HCl
  left) pH 4.0
• reach equivalence
  point
• --add 50.01mL 1.0M NaOH (0.0001 M NaOH
  left) pH 10.0
• --add 50.1mL 1.0M NaOH (0.001 M NaOH
  left) pH 11.0

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50.0mL 1.0M HCl 1.0M NaOH
pKa of phenolphthalein
pH
Equivalence point
mL base added
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Strong / Weak Acid / Base Titration Curve

• e.g. titration of 1.0M acetic acid with NaOH
• 1) starting pH (solution is acetic acid)
• C2H3O2H ltgt C2H3O2- H
• Start 1.0M 0 0
• At Equil. 1.0 - y y y
• y2/1.0M Ka 1.8 x 10-5 y
  H 4.24 x 10-3 pH 2.37
• 2) equivalence point (solution is acetate)
• C2H3O2- H2O ltgt C2H3O2H OH-
• Start 1.0M 0 0
• At Equil. 1.0 - y y y
• y2/1.0M Kb 7.1 x 10-10 y
  OH- 2.66 x 10-5
• pOH 4.57 pH 9.43

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Strong / Weak Acid / Base Titration Curve

• 3) Midpoint pH (solution is 5050 acetic acid
  acetate)
• C2H3O2H
• H Ka
• C2H3O2-
• C2H3O2H C2H3O2- , so pH pKa
  4.74
• Note that you have already done all three of
  these calculations.

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50.0mL 1.0M acetic acid 1.0M NaOH

note want indicator which will change color
in this region
starting pH
equivalence point
midpoint pH