Exploring Chemical Analysis 3e

1
______________________________________
Class, Wednesday, Dec 1, 2004 Announcements

• Changes from schedule in the syllabus
• pH and Spectrometer Labs due Friday, Dec 10.
• Last time we discussed Exam 3 on Friday, Dec
  10. Will you be able to work longer than an hour
  at that time, or do you wish to have Exam 3 on
  Wednesday, Dec 8? The class decided Wed, Dec 8
  for Exam 3. Ill finish lecturing on this
  Friday, so you have Mon and Fri of next week off.
• Final Exam, 1000 AM, Wed., Dec 15.

2
______________________________________
Titration Curve Problem for a weak acid

• 40.00 mL of a 0.03320 M alcohol and water
  solution of
• p-cresol is going to be titrated with a 0.05000 M
  solution of KOH. Select 5 or 6 meaningful points
  for this titration. Use these points to sketch
  the approximate titration curve to be expected.

3
______________________________________
Titration Curve Problem for a weak acid the
solution

• Let HCres represent the acid p-cresol. The
  chemical equation before the addition of any
  titrant may then be written as
• HCres H2O lt gt Cres- H3O
• and Ka Cres- H3O / HCres

4
______________________________________
Titration Curve Problem for a weak acid the
solution

• The first step is to calculate the volume of KOH
  titrant reach the equivalence point.
• At the equivalence point,
• the mmol of H mmol of OH-, or
• Vacid x Concacid Vbase x Concbase
• (40.00 mL)(0.03320 M) Vbase x 0.05000 M
• Vbase 26.66 mL, the volume of KOH at eq. pt.

5
______________________________________
Titration Curve Problem for a weak acid the
solution

• 2) The following points at selected to calculate
  the values of the pH at those points
• at VKOH 0.00, 8.00, 20.00, 26.66, 30.00,
  35.00 mL selection of the ½ V eq pt might also
  be helpful because at this point, the pH pKa

6
______________________________________
Titration Curve Problem for a weak acid the
solution

• 3) The pH values were calculated as described
  last lecture for the points selected. Also,
  recall from last lecture there are two points and
  two regions. They are
• Vo, weak acid
• V8.00 and V20.00, buffer region
• V26.66 , the eq pt, the weak conj base of the
  acid.
• V 30.00 and V35.00, region after eq pt,
  dilution of
• excess titrant
• Selection of the ½ V eq pt (1/2 x 26.66)
  13.33 might also be helpful because at this
  point, the pH pKa. The results of these
  calculations are shown in the table of the next
  slide.

7
______________________________________
Titration Curve Problem for a weak acid the
solution

8
______________________________________
Titration Curve Problem for a weak acid the
solution
Notes 1) This is not a great titration curve
because the break at the eq pt (25.66 mL) is so
small. 2) the change between V 0 and 8 is
large probably good to make an additional
calculation in this area, say at V 4 or 5 mL.

9
______________________________________
Titration Curve Problem for a Weak Base

10
______________________________________
Friday we will discuss the following problem
work on it on your own between now and then.

• The conjugate acid of the weak base cyclohexamine
  has a pKa 10.57. 30.00 mL of a 0.06000 M
  solution of cyclohexamine is to be titrated with
  a 0.04500 M solution of standard HCl. Find
  points (pH values) at Vo, 0.25 Vept, 0.50 Vept
  0.75 Vept , Vept , 1.1Vept and 1.2 Vept. Use
  these points to sketch a titration curve.