Answer : The pH of the solution is, 9.4
Solution : Given,
Concentration (c) = 0.025 M
Base dissociation constant = 
First we have to calculate the value of
.
The expression used for the calculation of
is,

Now put the value of
in this expression, we get:


The given equilibrium reaction is,

initially conc. 0.335 0 0
At eqm. (0.335-x) x x
Formula used :
![k_b=\frac{[C_5H_5NH^+][OH^-]}{[C_5H_5N]}](https://tex.z-dn.net/?f=k_b%3D%5Cfrac%7B%5BC_5H_5NH%5E%2B%5D%5BOH%5E-%5D%7D%7B%5BC_5H_5N%5D%7D)
Now put all the given values in this formula ,we get:

By solving the terms, we get:

Now we have to calculate the concentration of hydroxide ion.
![[OH^-]=x=2.4\times 10^{-5}M](https://tex.z-dn.net/?f=%5BOH%5E-%5D%3Dx%3D2.4%5Ctimes%2010%5E%7B-5%7DM)
Now we have to calculate the pOH.
![pOH=-\log [OH^-]](https://tex.z-dn.net/?f=pOH%3D-%5Clog%20%5BOH%5E-%5D)


Now we have to calculate the pH.

Therefore, the pH of the solution is, 9.4