Consider the following 0.1 N acids: I. Acetic acid II. Perchloric acid II, Chloroacetic acid IV. Hydrochloric acid What is the sequence of these in terms of their increasing acid strength in water?

The acid strength in water is determined by the degree of dissociation and the stability of the resulting conjugate base. Acetic acid (CH3COOH) is a weak organic acid with a pKa of approximately 4.76 [1]. Chloroacetic acid (ClCH2COOH) is stronger than acetic acid because the electronegative chlorine atom exerts an inductive effect (-I effect), stabilizing the carboxylate anion and lowering the pKa to about 2.86. Hydrochloric acid (HCl) and perchloric acid (HClO4) are both strong mineral acids that dissociate almost completely in water [1]. However, perchloric acid is considered the strongest known mineral acid due to the high oxidation state of chlorine and the resonance stabilization of the perchlorate ion, making it stronger than HCl [t1]. Therefore, the increasing order of acid strength is Acetic acid < Chloroacetic acid < Hydrochloric acid < Perchloric acid (I < III < IV < II).

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > 2.3 HOW STRONG ARE ACID OR BASE SOLUTIONS? > p. 26