Analytical Chem istry - DePauw University

450 Analytical Chemistry 2.0Practice Exercise 9.9Samples containing the monoprotic weak acids 2–methylaniliniumchloride (C 7 H 10 NCl, pK a = 4.447) and 3–nitrophenol (C 6 H 5 NO 3 ,pK a = 8.39) can be analyzed by titrating with NaOH. A 2.006-g samplerequires 19.65 mL of 0.200 M NaOH to reach the bromocresol purpleend point and 48.41 mL of 0.200 M NaOH to reach the phenolphthaleinend point. Report the %w/w of each compound in the sample.Click here to review your answer to this exercise.9B.5 Qualitative ApplicationsExample 9.5 shows how we can use an acid–base titration to assign theforms of alkalinity in waters. We can easily extend this approach to othersystems. For example, by titrating with either a strong acid or a strong baseto the methyl orange and phenolphthalein end points we can determine thecomposition of solutions containing one or two of the following species:H 3 PO 4 , H 2 PO 4 – , HPO 4 2– , PO 4 3– , HCl, and NaOH. As outlined in Table9.9, each species or mixture of species has a unique relationship betweenthe volumes of titrant needed to reach these two end points.9B.6 Characterization ApplicationsAn acid–base titration can be use to characterize the chemical and physicalproperties of matter. Two useful characterization applications are theTable 9.9 Relationship Between End Point Volumes for Mixtures of PhosphateSpecies with HCl and NaOHRelationship Between End Point Relationship Between End PointSolution CompositionVolumes with Strong Base Titrant a Volumes With Strong Acid Titrant aH 3 PO 4 V PH = 2 V MO— bH 2 PO 4–HPO 42–PO 43–V PH > 0; V MO = 0 —— V MO > 0; V PH = 0— V MO = 2 V PHHCl V PH = V MO—NaOH — V MO = V PHHCl and H 3 PO 4 V PH < 2 V MO—H 3 PO 4 and H 2 PO 4–H 2 PO 4–and HPO 42–HPO 42–and PO 43–V PH > 2 V MO—V PH > 0; V MO = 0 V MO > 0; V PH = 0— V MO > 2 V PHPO 43–and NaOH — V MO < 2 V PHa VPH and V MO are, respectively, the volume of titrant at the phenolphthalein and methyl orange end points.b When no information is provided, the volume of titrant to each end point is zero.