1. The problem statement, all variables and given/known data http://i.minus.com/jhPgz49XLoB3Z.png [Broken] 2. Relevant equations Strength of acid depends on conjugate stability. 3. The attempt at a solution Why can't the answer also be 4? Both have the same number of oxygens attached, each of which exerts an inductive effect. The only difference is one has Cl and the other has Br. One would think that the conjugate base of bromous acid would be more stable because despite Br's slight lack of electronegativity versus Cl (2.8 vs. 3.0), Br is much bigger than Cl, and is therefore able to spread out negative charge over a bigger surface area.