Correct Option (b):
Assertion (A) states that formic acid is a stronger acid than acetic acid. This statement is true. The relative strength of carboxylic acids is primarily determined by the stability of their conjugate bases. Formic acid (HCOOH) forms the formate ion (HCOO⁻) as its conjugate base, while acetic acid (CH₃COOH) forms the acetate ion (CH₃COO⁻).
The methyl group (–CH₃) in acetic acid exhibits a positive inductive effect (+I effect), meaning it donates electrons. This electron donation increases the electron density on the carboxylate oxygen in the acetate ion, thereby destabilizing it. In contrast, formic acid has only a hydrogen atom attached to the carboxyl group, which has a negligible inductive effect. Consequently, the formate ion is more stable than the acetate ion, making formic acid a stronger acid than acetic acid.
Reason (R) states that formic acid is an organic acid. This statement is also factually true, as formic acid contains carbon and is classified as an organic compound. However, this general classification does not explain why formic acid is stronger than acetic acid, especially since acetic acid is also an organic acid. The explanation for the difference in acid strength lies in the specific inductive effects of the groups attached to the carboxyl carbon, not in its broad category as an organic acid. Therefore, Reason (R) is true but is not the correct explanation for Assertion (A).
Incorrect Options:
Option (a) is incorrect because while both Assertion (A) and Reason (R) are true, Reason (R) does not provide the correct explanation for the assertion. The difference in acid strength is due to inductive effects, not the general classification as an organic acid.
Option (c) is incorrect because Reason (R) is factually true; formic acid is indeed an organic acid.
Option (d) is incorrect because Assertion (A) is true; formic acid is stronger than acetic acid due to the reasons explained above.