HCO₃⁻ + H₂O → H₂CO₃ + OH⁻ base + acid → Conj A + Conj B. We see that HCO₃⁻ becomes H₂CO₃. It has one more H atom and one more + charge (-1 + 1 = 0). So H₂CO₃ is the conjugate acid of HCO₃⁻. The H₂O becomes OH⁻. It has one less H atom and one more – charge. So OH⁻ is the conjugate base of H₂O. More @Wikipedia
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