Rules:
1. The more halogens are on your carboxylic acid, the more acidic it is
2. The closer the halogen is to the carboxylic acid group, the more acidic the molecule is.
Why? Halogens are electronegative. They withdraw charge from the bond, making the carbon they are attached to partially positive. The more acidic an acid is, the easier it is for it to loose a hydrogen, leaving negatively charged molecule behind. Thus the more stabilized this negatively charged molecule, the better. Halogens stabilize the negative charge by balancing it with the partially positive charge that they create on the carbon they are attached to.