Answer
The protonation of the other oxygen is shown in the picture. Protonation of the double bonded oxygen is favored because it forms more resonance structures after protonation.
Work Step by Step
The protonation of the other oxygen is shown in the picture. When the double bonded oxygen is protonated there are three resonance forms formed. However, the protonation of the other oxygen does not form a structure stabilized by resonance. Thus, the protonation of the double bonded oxygen is favored because it forms more resonance structures after protonation.
