Question

Among the following acids which has the lowest $p{k_a}$ ​ value?
(A) $C{H_3}C{H_2}COOH$
(B) ${(C{H_3})_2}CH - COOH$
(C) $HCOOH$
(D) $C{H_3}COOH$

Answer 1

Hint : The $p{k_a}$ value is a measure to indicate the strength of an acid. It is the negative log of the value of acid dissociation constant i.e. ${k_a}$ . The dissociation constant is the measure of the strength of an acid in a chemical solution. In equilibrium, it is the equilibrium constant.

Complete answer:
$p{k_a}$ values are used to determine the acidity of acid and base. Lower $p{k_a}$ Values indicate that the dissociation constant value of the acid is very high, i.e. the acid will dissociate into ${H^ + }$ faster. It means the acid will be more acidic. Therefore, we have to check the dissociation rate of ${H^ + }$ ion of the given acids and determine which acid has the lowest $p{k_a}$ value and will be more acidic.
The presence of an alkyl group affects the acidity of the acids. Alkyl groups show an inductive effect and due to the inductive effect the electron density on the Oxygen atom increases which hinders the cleavage of $O - H$ bond. More the number of alkyl groups more will be the inductive effect and less will be the dissociation rate of $O - H$ bond. Hence, the acid will have a higher ionization constant i.e. higher $p{k_a}$ values. Now out of the given options, only formic acid $HCOOH$ is the only compound that does not have any alkyl group in it so, it will dissociate ${H^ + }$ easily and due to greater ionization, it will have the lowest $p{k_a}$ value among all the other acids.
Hence, option (C) is correct.

Note:
Lower $p{k_a}$ value means higher ${k_a}$ values i.e. higher ionization constant. It means higher will be the acidity of the acid. Hence the compounds that have low $p{k_a}$ values tend to be more acidic than those having higher $p{k_a}$ values.