Part A
Pyridine is a weak base that is used in the manufacture of pesticides and plastic resins. It is also a component of cigarette smoke. Pyridine ionizes in water as follows:
C5H5N+H2O⇌C5H5NH++OH−
The pKb of pyridine is 8.75. What is the pH of a 0.110 M solution of pyridine? (Assume that the temperature is 25 ∘C.)
use:
pKb = -log Kb
8.75-log Kb
Kb = 1.778*10^-9
C5H5N dissociates as
C5H5N + H2O -----> C5H5NH+ + OH-
0.11 0 0
0.11-x x x
Kb = [C5H5NH+][OH-]/[C5H5N]
Kb = x*x/(c-x)
Assuming x can be ignored as compared to c
So, above expression becomes
Kb = x*x/(c)
so, x = sqrt (Kb*c)
x = sqrt ((1.778*10^-9)*0.11) = 1.399*10^-5
since c is much greater than x, our assumption is correct
so, x = 1.399*10^-5 M
use:
pOH = -log [OH-]
= -log (1.399*10^-5)
= 4.8543
use:
PH = 14 - pOH
= 14 - 4.8543
= 9.1457
Answer: 9.15
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