Lehninger Principles of Biochemistry 6th Edition

Published by W.H. Freeman
ISBN 10: 1-42923-414-8
ISBN 13: 978-1-42923-414-6

Chapter 2 - Water - Problems - Page 72: 17

Answer

Answer: pH of resulting solution is 7.4

Work Step by Step

*First use Henderson-Hasselbalch equation to calculate the ratio of conjugate base to acid in the starting solution: pH = pKa + log ([A-]/[HA]) 8.0 = 7.4 + log ([A-]/[HA]) 0.6 = log ([A-]/[HA]) [A-]/[HA] = 10^0.6 = 4 *Solution contains 100 meq of compound (conjugate base plus acid), thus 80 meq is in conjugate base form and 20 meq is in the acid form, with a [base]/[acid] ratio of 4. *HCl is a strong acid and dissociates completely. Thus, adding 30 mL of 1.0 M HCl will add 30 meq of H+ to the solution. These new 30 meq titrate 30 meq of the conjugate base, resulting in a [base]/[acid] ratio of 1. *Last, solve the Henderson-Hasselbalch equation for pH (given pKa 7.4): pH = pKa + log ([A-/[HA]) = 7.4 + log (1) = 7.4 + 0 = 7.4 Answer: pH of resulting solution is 7.4
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