The acid strength of saturated aliphatic carboxylic acids depends mainly upon the inductive effect of the substituent and its position w.r.t. the -COOH group. Where as electron donating substituents tend to increase the acid strength. The acidic strength of aromatic carboxylic acids, on the other hand, depends upon both the inductive and the resonance effect of the substituents.

Which of the following would be expected to be most highly ionized in water?

CH₃CH₃CCl₂COOH

Fast ionization is shown by strong acids. Chlorine atom has high $-I$ effect so when attached to nearly carbon, it increases acidity thus is more acidic.