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Titration Curves and Exam Schedule for Chemistry Class

In this class announcement from December 1, 2004, students are informed about changes to the schedule. Key details include labs due on December 10, the decision to hold Exam 3 on December 8, and the final exam on December 15. The announcement discusses a titration curve problem regarding the titration of p-cresol with KOH, detailing required calculations and selected points for analysis. Students are also advised to prepare for the next class by working on a related problem involving cyclohexamine and HCl titration.

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Titration Curves and Exam Schedule for Chemistry Class

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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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