The conjugate base of H3PO4 (phosphoric acid) is H2PO4– (dihydrogen phosphate ion).
Here’s why: In an acid-base reaction, an acid donates a proton (H+) to become its conjugate base. Phosphoric acid (H3PO4) can lose one proton to form H2PO4–. This molecule now has one less hydrogen ion and carries a negative charge. Thus, when phosphoric acid acts as an acid and donates a proton, H2PO4– is the resulting conjugate base.