The conjugate base of H2CO3 (carbonic acid) is HCO3- (bicarbonate ion).
When carbonic acid (H2CO3) loses a proton (H+), it forms the bicarbonate ion (HCO3-). This process is represented by the following chemical equation:
H2CO3 → HCO3- + H+
In this reaction, H2CO3 acts as an acid by donating a proton, and HCO3- is its conjugate base. The bicarbonate ion can further lose a proton to form the carbonate ion (CO3^2-), but that would be the conjugate base of HCO3-, not H2CO3.