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 acid has the smallest dissociation constant?

BrCH₂CH₂COOH

BrCH₂CH₂COOH has the smallest dissociation constant ( i.e., is the weakest acid) since Br has lower -I effect than F and is far removed from the -COOH group.