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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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  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. 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.

  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 < == > 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.

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