Answer the question on basis of passage given below:

Carboxylic acids are weaker than mineral acids, but they are stronger acids than alcohols and many simple phenols (\(pK_a\) is ∼16 for ethanol and 10 for phenol) The higher acidity of carboxylic acids as compared to phenols can be understood similarly. The conjugate base of carboxylic acid, a carboxylate ion, is stabalized by two equivalent resonance structures in which the negative charge is at the more electronegative oxygen atom. The conjugate base of phenol,a phenoxide ion, has non-equivalent resonance structures in which the negative charge is at the less electronegative carbon atom. Further, the negative charge is delocalized over two electronegative oxygen atoms in carboxylate ion whereas it is less effectively delocalized over one oxygen atom and less electronegative carbon atoms in phenoxide ion.Thus, the carboxylate ion is more established than phenoxide ion, so carboxylic acids are more acidic than phenols.

Effect of substituents on the acidity of carboxylic acids: substituents may affect the stability of the conjugate base and thus, also affect the acidity of the carboxylic acids. Electron withdrawing group increase the acidity of carboxylic acids by stabilizing the conjugate base through delocalization of the negative charge by by inductive and /or resonance effects. Conversely, electron donating groups decrease the acidity by destabilising the conjugate base.

The weakest among the following acid is:

Benzoic acid

The correct answer is option 2. Benzoic acid.

The acidity of a carboxylic acid is determined by how easily it can donate a proton (H⁺) to form a carboxylate ion (\(-COO^-\)). Factors influencing acidity include:

Electronegative Substituents: Groups that withdraw electrons from the carboxyl group (electron-withdrawing groups, EWGs) stabilize the carboxylate ion, making the acid stronger.

Resonance Effects: Substituents that stabilize the negative charge on the carboxylate ion through resonance also increase acidity.

Analysis of Each Acid:

1. Formic Acid (\(HCOOH\)):

The simplest carboxylic acid with no additional substituents. Formic acid has a \( pK_a \) of about 3.75. The absence of strong electron-withdrawing or donating groups means its acidity is relatively moderate compared to other substituted acids.

2. Benzoic Acid (\(C_6H_5COOH\)):

Contains a phenyl group attached to the carboxyl group. Benzoic acid has a \( pK_a \) of approximately 4.2. The phenyl group provides some resonance stabilization to the carboxylate ion, making benzoic acid more acidic than formic acid. However, this effect is not as strong as the effects provided by electron-withdrawing groups.

3. Nitroacetic Acid (\(O_2NCH_2COOH\)):

Contains a nitro group attached to the \(-CH_2\) group. Nitroacetic acid is a much stronger acid with a \( pK_a \) significantly lower than formic and benzoic acids. The nitro group is a powerful electron-withdrawing group through both inductive and resonance effects, greatly stabilizing the carboxylate ion and thus increasing the acidity of the acid.

4. Trichloroacetic Acid (\(CCl_3COOH\)):

Has three chlorine atoms attached to the \(-CH_2\) group. Trichloroacetic acid is an extremely strong acid with a \( pK_a \) of about 0.7. The three chlorine atoms are very strong electron-withdrawing groups that stabilize the carboxylate ion very effectively, making this acid much stronger than the other acids listed.

Comparative Summary:

Formic Acid: Moderate acidity with a \( pK_a \approx 3.75\).

Benzoic Acid: Slightly less acidic than formic acid due to weaker electron-withdrawing effects; \( pK_a \approx 4.2\).

Nitroacetic Acid: Strong acid due to the powerful electron-withdrawing nitro group, with a much lower \( pK_a \).

Trichloroacetic Acid: Very strong acid with a very low \( pK_a \) due to the strong electron-withdrawing effects of three chlorine atoms.

Conclusion: Benzoic Acid is the weakest acid among the given options. It has the highest \( pK_a \) value, indicating it is the least acidic compared to formic acid, nitroacetic acid, and trichloroacetic acid.

Thus, the weakest acid is: 2.  Benzoic Acid