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,

hydrocarbons are organic compounds that contain only hydrogens and carbons. they can be saturated (those which contain only carbon-carbon single bonds) or unsaturated (those which contain carbon-carbon double or triple bonds) in nature. hydrocarbons can undergo several reactions like substitution, elimination etc.  

on substituting one or more hydrogen atoms in hydrocarbon it results in the formation of haloalkane. as the halogen atoms are large compared to the carbon and hydrogen atoms, due to which the molecular weight increases and the bond become polar because of presence electronegative halogen atom and results in the increase in boiling point of the haloalkane.

thus, the correct option is (b) that is the boiling point of the new compound increases on substituting a hydrogen atom with a halogen in a hydrocarbon.