Answer:
The pH of the solution is 2.56.
Explanation:
Given :
Concentration (c) = 0.40 M
Acid dissociation constant = 
The equilibrium reaction for dissociation of
(weak acid) is,

initially conc. c 0 0
At eqm.

Dissociation constant is given as:


By solving the terms, we get value of 

No we have to calculate the concentration of hydronium ion or hydrogen ion.
![[H^+]=c\alpha=0.4\times 0.00686832=0.002747 M](https://tex.z-dn.net/?f=%5BH%5E%2B%5D%3Dc%5Calpha%3D0.4%5Ctimes%200.00686832%3D0.002747%20M)
Now we have to calculate the pH.
![pH=-\log [H^+]](https://tex.z-dn.net/?f=pH%3D-%5Clog%20%5BH%5E%2B%5D)


Therefore, the pH of the solution is, 2.56.