Class, Friday, Oct 24

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Class, Friday, Oct 24
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Announcements

• Homework Ch 8 originally due on Monday, Oct 27
  moved to Wednesday, Oct 29
• Calcium lab due at classtime on Friday, Oct 31
• I am changing the date of exam 2 to Monday, Nov 3
  unless there are problems
• No class on Monday, Oct 27 Prof and Chris will
  be attending the regional American Chemical
  Society meeting where Chris is giving a poster
  presentation. Congratulations to Chris!

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Chapter 9 Acid-Base Titrations
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The titration of femtomolar (10-15) quantities of
nitric acid.
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The titration of a Strong Base with a Strong Acid

• Chemical Reaction net ionic equation
• H OH? ? H2O
• The specific identity of the acid and base is
  not important for this titration.

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The titration of a Strong Base with a Strong Acid

• The equivalence point is whenever the
• number of moles of the titrant are exactly equal
• to the number of moles of the analyte.
• The end point is that pH when the visual
  indicator
• changes color as discussed in chapter 8. For
• accurate titrations, the end point ought to be
  the
• same as the equivalance point. If it is not, the
• difference is the end-point error.

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The titration of a Strong Base with a Strong Acid

• The titration curve (plot of pH vs Volume of
  titrant) may be calculated for the SA/SB
  titration by considering 3 unique regions They
  are
• Region before the equivalence point
• The equivalence point
• Region after the equivalence point
• We want to consider each of the above points. We
  can
• calculate the pH values as a function of the
  amount of added
• titrant.

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The titration of a Strong Base with a Strong Acid

• Region before the equivalence point
• Before the equivalence point the unreacted OH?
  establishes the pH of the reaction mixture. The
  calculations involved determining the number of
  moles of OH- minus the number of moles of H
  added to that point, and dividing by the volume
  of the reaction mixture, generally the initial
  volume plus the volume of the titrant added.

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The titration of a Strong Base with a Strong Acid

• Region before the equivalence point
• For example, what is the pH after 10.00 mL of
  0.1000 M HCl has been added in the titration of
  50.00 mL of 0.1000 M NaOH?
• Initial mole OH- 50.00 x 0.1000 5 x 10-3 or
  5 mmol
• mole H added 10.00 x 0.1000 1 x 10-3 or 1
  mmol
• moles unreacted OH- 5 1 4 mmol
• Total volume 10.00 50.00 60.00 mL
• OH- 4mmol / 60.00mL 0.06667 mmol/mL
• pOH -log(0.06667) 1.18 pH 12.82
• All other points prior to the equivalence point
  are calculated by like fashion.

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The titration of a Strong Base with a Strong Acid

• At the equivalence point
• For the SA/SB titration, the pH at the
  equivalence point is always 7.00.
• Often we will need to calculate the volume of
  titrant to reach the equivalence point. At the
  equivalence point the mol of OH- mol H.
• In the previous example, the total mol of base
  were 5 mmol therefore, 5 mmol of acid are
  required to reach the equivalence point. Since V
  x M mol, 5mmol / conc volume of titrant, or
  in this case, 5 / 0.1000 50.00 mL, the volume
  of HCl to reach the equivalence point

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The titration of a Strong Base with a Strong Acid

• Region after the equivalence point
• The pH of the reaction mixture after the
  equivalence point is established by the
  concentration of the excess amount of titrant
  (here the acid)
• What is the pH of the solution whenever a total
  of 55 mL of HCl has been added?
• The mmol of H in excess (55 50)(0.1000)
• 0.5000 mmol
• The total volume at this point is (50 55)
  105mL
• The H 0.5000 mmol/105 mL 0.00476 M
• pH -log(0.00476) 2.32

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The titration curve for the titration of a strong
base such as NaOH with a strong acid such as HCl
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