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-
