The pH of a 0.78M solution of 4-pyridinecarboxylic acid HC6H4NO2is measured to be 2.53.Calculate the acid dissociation constant Ka of 4-pyridinecarboxylic acid. Round your answer to 2 significant digits.

Answer : The value of acid dissociation constant is,

Solution :  Given,

Concentration pyridinecarboxylic acid = 0.78 M

pH = 2.53

First we have to calculate the hydrogen ion concentration.

Now we have to calculate the acid dissociation constant.

The equilibrium reaction for dissociation of (weak acid) is,

initially conc.         0.78                0             0

At eqm.               (0.78-x)              x             x  

The expression of acid dissociation constant for acid is:

As,

So,  

Now put all the given values in this formula ,we get:

Therefore, the value of acid dissociation constant is,

i think   the answer is c hope i'm right