Chemistry: The Molecular Nature of Matter and Change 7th Edition

Published by McGraw-Hill Education
ISBN 10: 007351117X
ISBN 13: 978-0-07351-117-7

Chapter 19 - Problems: 19.33

Answer

Thus, the acid-base pairs astisfying the given conditions are : (part a) 1. HOOC(CH2)4COOH / HOOC(CH2)4COO- 2. C6H5NH3+ / C6H5NH2 (part b) 1. H2AsO4-/(HAsO4)2- 2. (H2PO3)- /(HPO3) 2-

Work Step by Step

1 Concept : The Henderson - Hasselbalch equation of a buffer is as follows : pH = pKa + log [conjugate base] / [acid] i.e. pH = pKa + log([A-] / [HA]) In order to determine the Acid-Conjugate Base pair, we can assume the concentrations to be equal. If [A-] = [HA], log([A-] / [HA]) = log 1 = 0 So, pH = pKa 2 Solution : Part A : pKa = pH = 4.5 -log(Ka) = 4.5 Ka = 3.16 x 10^-5 Looking at Appendix C, we can conclude that the appropriate Acid for the required Ka is, HOOC(CH2)4COOH/HOOC(CH2)4COO- Similarly, pH = 4.5 pOH = 14 - pH = 14 - 4.5 = 9.5 And, pOH = pKb = -log(Kb) So, -log(Kb) = 9.5 Kb = 3.16 x 10^-10 Looking at Appendix C, we can conclude that the appropriate Acid for the required pH is, C6H5NH3+/C6H5NH2 3 Part B : pH = pKa = 7 -log(Ka) = 7 Ka = 1.0 x 10^-7 Thus, the acid base pairs satisfying the given conditions are : H2AsO4-/(HAsO4)2- OR (H2PO3)- /(HPO3) 2-
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