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Class, Wednesday, Dec 1, 2004 Announcements. ______________________________________. Changes from schedule in the syllabus: pH and Spectrometer Labs due Friday, Dec 10.

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slide1

Class, Wednesday, Dec 1, 2004

Announcements

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  • 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. I’ll finish lecturing on this Friday, so you have Mon and Fri of next week off.
    • Final Exam, 10:00 AM, Wed., Dec 15.
slide2

Titration Curve Problem for a weak acid

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  • 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.
slide3

Titration Curve Problem for a weak acid: the solution

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  • Let HCres represent the acid p-cresol. The chemical equation before the addition of any titrant may then be written as
  • HCres + H2O < == > Cres− + H3O+
  • and Ka = [Cres− ] [H3O+] / [HCres]
slide4

Titration Curve Problem for a weak acid: the solution

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  • 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.
slide5

Titration Curve Problem for a weak acid: the solution

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  • 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
slide6

Titration Curve Problem for a weak acid: the solution

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  • 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.
slide8

Titration Curve Problem for a weak acid: the solution

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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.

slide10

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

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  • 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.