Question

Percent lonization of a Weak Acid 7 of 13 Constants I Periodic Table 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: CsHN+H20C,HNH+OH (Assume that the temperature is 25° C.) The pK of pyridine is 8.75. What is the pH of a 0.325 M solution of pyridine? Express the pH numerically to two decimal places. View Available Hint(s) VAED ? pH Submit Part B

Answer 1

A)

use:

pKb = -log Kb

8.75= -log Kb

Kb = 1.778*10^-9

C5H5N dissociates as:

C5H5N +H2O -----> C5H5NH+ + OH-

0.325 0 0

0.325-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)*0.325) = 2.404*10^-5

since c is much greater than x, our assumption is correct

so, x = 2.404*10^-5 M

So, [OH-] = x = 2.404*10^-5 M

use:

pOH = -log [OH-]

= -log (2.404*10^-5)

= 4.6191

use:

PH = 14 - pOH

= 14 - 4.6191

= 9.3809

Answer: 9.38