Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods531values as a guide, the pH at the first equivalence point is between a pHof 2.97 and 13.74, and the second equivalence points is at a pH greaterthan 13.74. From Table 9.4, phenolphthalein’s end point falls in the pHrange 8.3–10.0. The titration, therefore, is to the first equivalence pointfor which the moles of NaOH equal the moles of salicylic acid; thus0.1354molNaOHL× 0. 02192 L= 2.968× 10 −3mol NaOH1 molC HO32. 968× 10−7 6 3molNaOH× ×molNaOH13812. gC HO7 6 3= 0.4099 gC HOmolC HO7 6 30.4099 gC HO7 6 3 × 100 = 97. 41%w/w CHO0.4208 gsample7 6 37 6 3Because the purity of the sample is less than 99%, we reject the shipment.Click here to return to the chapter.Practice Exercise 9.7The moles of HNO 3 produced by pulling the air sample through thesolution of H 2 O 2 is0.01012 molNaOH× 0.00914 L×L1 molHNO3= 925 . × 10 −5molHNOmolNaOH3A conservation of mass on nitrogen requires that each mole of NO 2 inthe sample of air produces one mole of HNO 3 ; thus, the mass of NO 2in the sample is51 molNO925 . × 10−2molHNO × ×3molHNO46.01 gNOmolNOand the concentration of NO 2 is426 . × 10− 35 LairgNOClick here to return to the chapter.2223= 426 . × 10 −3gNO1000 mg× = 0.852 mg NO /L air2g2