This is an acidic proton located on a water molecule which is co-ordinated to a metal ion. Example:
2. Hydroxoacid
This is an acidic proton on a hydroxyl (OH) group, which does not have a neighboring oxo group (=O). An example is telluric acid:
Which is a white solid that dissolves in water.
3. Oxoacid
This is an acidic proton on a hydroxyl group, which has a neighboring oxo group. The most common ones are mononuclear, examples:
Often one or more of the OH groups can be replaced with an electron-withdrawing group such as F or CF3, or an electron-donating group such as NH2 - which can donate electrons by pi-resonance. Charge is more stable when spread out, so we can increase acidity by making an anion more stable by delocalizing charge (ie. pulling it off the oxygen atom).
Not all oxoacids follow the simple pattern of one atom surrounded by =O and -OH. For example, phosphonic acid (H3PO3) is only diprotic. Its easy to lose a mark in an exam by forgetting that one of the hydrogens isn't attached to an OH:
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