Please Pick Up

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Please Pick Up

• pH and Aqueous Solutions Problem Set

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pH and Titration Methods

• Edward A. Mottel
• Department of Chemistry
• Rose-Hulman Institute of Technology

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pH and Titration Methods

• Reading assignment
• Chang Chapter 4.5, 4.7, 16.2-16.4

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Autoionization of Water

• Water is a very weak electrolyte

Dissociation constant Kw H OH 1.0 x
1014 (_at_ 25 C)
Note H(aq) º H3O (aq)
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pH

• Base 10 logarithmic scale of hydrogen ion
  concentration
• pH -log H

Be careful use log and not ln
If pH -log H , what does H equal?
H 10pH
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pH
Whats the pH of pure water?
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pH
If the pH of an aqueous solution is 3.50 at 25
C, what is the hydroxide ion concentration?
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pH
H 10pH
H 103.50 3.2 x 104 M
Kw H OH 1.0 x 1014 M2
OH Kw / H
OH (1.0 x 1014 M2) / 3.2 x 104 M
3.2 x 1011 M
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pH
pH values can be outside 0-14
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Titration
buret
an analytical technique to determine the
concentration of a solution
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Thermometric Titration
acid-base reactions are usually exothermic
T (C)
Vacid
Moles of acid Moles of base
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Titration
an analytical technique to determine the
concentration of a solution
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Titration
the purpose of a titration is to determine
the equivalence point
Moles of acid Moles of base
nacid Macid Vacid nbase Mbase Vbase
some have two (H2SO4),
some have three (H3PO4)
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Titration
Moles of acid Moles of base
nacid Macid Vacid nbase Mbase Vbase
some accept two (Ca(OH)2),
some accept three (Al(OH)3)
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Equivalence Point Detection

• Physical methods
• Thermometric
• Potentiometric
• Visual indicator
• Conductometric

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Potentiometric Titration 20.0 mL of 0.400 M HCl
with Aqueous Ammonia
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Equivalence Point Detection

• Visual indicator methods
• Color change
• Endpoint versus equivalence point
• Endpoint when the indicator changes color
• Equivalence point
• moles of acid moles of base
• Factors to consider when selecting an indicator

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Bromocresol Green
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Common Acid-Base Visual Indicators
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Potentiometric Titration 20.0 mL of 0.400 M HCl
with Aqueous Ammonia
phenolphthalein
bromocresol green
For this titration, which indicator would be
better?
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Conductometric Titration
Depending on the acid-base system studied, the
graph may have many different shapes.
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Conductometric Titration
Why does the line change slope?
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Conductometric Titration
Total
K
OH
H
Cl
Ions
100
20
50
30
0
20
100
40
100
60
120
80
160
100
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Conductometric Titration
If the total number of ions isnt changing before
the equivalence point, why is the conductivity
changing?
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Laboratory Schedule forThis Week and Next Week

• A standardized KOH solution will be provided.
• Determine the molarity of the acid (HCl) using by
  indicator titration with the KOH. (two
  indicators)
• Determine the properties of an unknown solution.
  It could be an acid, a base or a salt.
• Analyze the unknown solution using the pH,
  conductometric and indicator titration with
  either the HCl or KOH (as appropriate).