Identify the acid with highest pKa value

The correct answer is Option (1) → HCOOH

The strength of a carboxylic acid depends on the stability of its carboxylate ion. An electron withdrawing group (−I effect) near the –COOH group spreads out the negative charge, stabilises the anion and increases acidity. The effect is stronger for a more electronegative atom: F > Cl > Br.

A stronger acid has a lower pK_a (pK_a = −log K_a), so the weakest acid has the highest pK_a.

    pK_a: about 2.6 < 2.86 < 2.90 < 3.75

HCOOH carries no halogen, so it has no −I group to help it. It is the weakest of the four acids and has the highest pK_a (3.75).

Why the other options are incorrect

• Option (2) BrCH₂COOH — Incorrect. The −I effect of Br makes it a stronger acid (pK_a 2.90).

• Option (3) FCH₂COOH — Incorrect. F is the most electronegative, so this is the strongest acid here, with the lowest pK_a.

• Option (4) ClCH₂COOH — Incorrect. The −I effect of Cl makes it a stronger acid (pK_a 2.86).

NCERT (2026-27 edition):
Reactions Involving Cleavage of O–H Bond – “… 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 groups increase the acidity of carboxylic acids by stabilising the conjugate base through delocalisation of the negative charge by inductive and/or resonance effects. …”