Weak Acids: Titration Curves, Buffers, and the HendersonHasselbach Equation

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Weak Acids Titration Curves, Buffers, and the
Henderson-Hasselbach Equation

• Experiment 13

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13 Weak Acids Titration Curves, Buffers, and
the Henderson-Hasselbach Equation

• Goal
• Use titration pH curve and melting point to
  identify an unknown weak organic acid
• Method
• Find nacid, MMacid, Ka and from titration of acid
  solution
• Find melting point range of solid acid from
  melting point experiments

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Acid-Base Definitions

• Acids
• generate H in water
• H donors
• excess H
• Bases
• generate OH- in water
• H acceptors
• Excess OH-

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Equilibrium in Water
Small K ? equilibrium favors reactants
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As H rises, OH- falls
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H and pH
H 1 ? 100 to 1 ? 10-14 in water pH
1 to 14 in water
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Relationships
H
OH-
H2O
H gtOH-
H OH-
H ltOH-
Basic solution
Acidic solution
Neutral solution
pH gt 7
pH lt 7
pH 7
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Strong Acids (exp. 7)

• 100 dissociation / good H donor
• equilibrium lies far to right
• HA ? H A-

Before dissociation
After dissociation
Relative moles
HCl H Cl-
HCl H Cl-
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Weak Acids (exp. 13)

• lt100 dissociation / not-as-good H donor
• equilibrium lies far to left
• HA H A-

Before dissociation
After dissociation
Relative moles
HA H A-
HA H A-
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Acid Dissociation Constant

• For a weak acid

?amount dissociated
?amount undissociated
10-2 lt Ka lt 10-7 2 lt pKa lt 7
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Henderson-Hasselbach Equation
Smaller Ka? weaker acid Larger pKa? weaker acid
log(xy) log x log y log(x/y) log (y/x)
For buffer system only considerable HA,A-
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H, pH and Ka, A-, HA

• A- lt HA ?
• A- gt HA ?
• A- HA ?

? pH lt pKa ? pH gt pKa ? pH pKa
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Chemical Equations

• 1) Weak acid HA dissociation
• 2) Reverse of water autoionization

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Chemical Equations
3. HA neutralization with strong base, NaOH
Ka
1/Kw
Kneutralization
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Strong acid-strong base titrations

• At equivalence point, Veq

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Weak acid-strong base titrations
pH

• equiv. point

VNaOH
½ Vequiv
Vequiv
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Differences in pH curves
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Buffer Characteristics

• Contain relatively large amounts of weak acid and
  corresponding base.
• Added H reacts to completion with weak base, A-.
• Added OH? reacts to completion with weak acid,
  HA.
• pH is determined by ratio of concentrations of
  weak acid and weak base.

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Key points on the pH curve

• 1) ½ Vequil
• 2) Vequil

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Molar Mass of HA

• Using titration data

And mass of HA used (analytical balance)
Find molar mass of HA
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Part 1 Preparation of Acid Solution
Dissolve solid acid

• Use 0.3000 g acid (record acid )
• Dissolve
• a 20 mL alcohol /80 mL H2O
• w 100 mL H2O

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Part 1 Titration Curve of HA with NaOH

• Record M NaOH MNaOH
• Add NaOH
• up to pH gt 11
• Record pHmeter pH
• during titration
• Two trials
• Draw pH curve
• Calculations nHA,pKa,MMHA

NaOH titrant
Acid
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Part 2 Melting Point Determination

• TAs will instruct how to do this in lab
• Each sample can only be melted ONCE
• Record melting range
• 132 138C Tfirst liquid to Tall
  liquid
• or 120 126C

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Data

• Find ½Vequiv ,Vequiv from pH curve ½Vequiv
• Vequiv
• Determine pKa from ½Vequiv pKa
• Determine nHA from Vequiv nHA
• Calculate MMHA from nHA and massHA MMHA
• Identify HA from table
• Use molar mass and melting point HA

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Table of possible acids
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Example Titration Curve

• ½Veq 8.6 mL
• pH 3.5
• pKa 3.5
• Veq 17.2 mL
• pH 7.8
• nHA 1.710-3

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Example Data

• And suppose at ½ Veq pH 3.5 ? pKa ? 3.5
• Veq pH ? 7.8

Consistent with acetylsalicylic acid
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Does neutralization go to completion?
HA neutralization with strong base, NaOH
Ka
1/Kw
Kneutralization
Goes to completion (107 lt Kneutlt1012)
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Report

• Abstract
• Results including
• Titration curves
• Molar mass of HA, pKa, Tmelting
• Identify acid
• Sample calculations
• Moles OH-, moles HA
• Molar mass of HA
• Discussion/review questions

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Example from text