The conjugate base of HClO4 (perchloric acid) is ClO4– (perchlorate ion).
When HClO4 donates a proton (H+), what remains is the chloride ion. In acid-base chemistry, this is how conjugate pairs work: the acid loses a proton to become its conjugate base. Therefore, for HClO4, when it ionizes, it forms ClO4–, which is its conjugate base. This relationship is critical in understanding the behavior of acids and bases in various chemical reactions.