Conjugate base of acid H3PO4=

I have H2PO-4

Is this correct?

If you meant H2PO4^-, yes.

Yes, your answer is correct. The conjugate base of an acid is formed by removing a proton (H+) from the acid molecule. In the case of H3PO4 (phosphoric acid), the conjugate base is formed by removing one proton.

To determine the conjugate base, you need to identify the acid's formula and then remove the appropriate number of protons. H3PO4 has three protons, and by removing one proton, you get H2PO-4, which is the conjugate base of H3PO4.