[Al(H2O)6]3+ + H2O ⇋ [Al(H2O)5(OH)]2+ + H3O+ Ka ~10-5 [1] cf acetic acid The OH- is bound as a ligand to the Al3+ cation thus the complex has an overall charge of 2+. So, The conjugate base is [Al(H2O)5(OH)]2+
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