Analytical Chem istry - DePauw University

530 Analytical Chemistry 2.0pHFigure 9.48 Titration curve for Practice Exercise9.3. The black dots and curve are theapproximate sketch of the titration curve.The points in red are the calculations fromPractice Exercise 9.2.pH14121086420141210864200 20 40 60 80 100Volume of HCl (mL)0 20 40 60 80 100Volume of NaOH (mL)Figure 9.49 Titration curve for Practice Exercise9.4. The black points and curve arethe approximate titration curve, and the redcurve is the exact titration curve.Practice Exercise 9.3Figure 9.48 shows a sketch of the titration curve. The two points beforethe equivalence point (V HCl = 5 mL, pH = 10.24 and V HCl = 45 mL,pH = 8.24) are plotted using the pK a of 9.244 for NH 4 + . The two pointsafter the equivalence point (V HCl = 60 mL, pH = 2.13 and V HCl = 80 mL,pH = 1.75 ) are from the answer to Practice Exercise 9.2.Click here to return to the chapter.Practice Exercise 9.4Figure 9.49 shows a sketch of the titration curve. The titration curve hastwo equivalence points, one at 25.0 mL (H 2 A HA – ) and one at 50.0mL (HA – A 2– ). In sketching the curve, we plot two points before thefirst equivalence point using the pK a of 3 for H 2 AV HCl = 2.5 mL, pH = 2 and V HCl = 22.5 mL, pH = 4two points between the equivalence points using the pK a of 5 for HA –V HCl = 27.5 mL, pH = 3, and V HCl = 47.5 mL, pH = 5and two points after the second equivalence pointV HCl = 70 mL, pH = 12.22 and V HCl = 90 mL, pH = 12.46)Drawing a smooth curve through these points presents us with the followingdilemma—the pH appears to increase as the titrant’s volume approachesthe first equivalence point and then appears to decrease as itpasses through the first equivalence point. This is, of course, absurd; aswe add NaOH the pH cannot decrease. Instead, we model the titrationcurve before the second equivalence point by drawing a straight line fromthe first point (V HCl = 2.5 mL, pH = 2) to the fourth (V HCl = 47.5 mL,pH = 5), ignoring the second and third points. The results is a reasonableapproximation of the exact titration curve.Click here to return to the chapter.Practice Exercise 9.5The pH at the equivalence point is 5.31 (see Practice Exercise 9.2) andthe sharp part of the titration curve extends from a pH of approximately7 to a pH of approximately 4. Of the indicators in Table 9.4, methyl redis the best choice because it pK a value of 5.0 is closest to the equivalencepoint’s pH and because the pH range of 4.2–6.3 for its change in colorwill not produce a significant titration error.Click here to return to the chapter.Practice Exercise 9.6Because salicylic acid is a diprotic weak acid, we must first determineto which equivalence point it is being titrated. Using salicylic acid’s pK a