What is the conjugate acid for H2O (water)?
As a general rule "rough method" , add a proton (H) to make conjugate acid. Remove a proton to make a conjugate base.
The conjugate acid is simply the original species PLUS a proton,
In both instances, mass and charge are conserved. If we add a proton to water we get
Given this, you know that the conjugate base for water is