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I'm a student in highschool AP chemistry. I'm confused about a certain problem in my chem textbook. In summary, I am given a species and asked to identify its basicity as strong, weak, or negligible. I must then give its conjugate acid and identify likewise for its acidity. (in the context of Brønsted-Lowry theory)

I understood all parts of the problem except for $\mbox{O}^{2-}$, to which I assigned the conjugate acid as $\mbox{OH}^{-}$. It was correct thus far. My guess that they were both weak was wrong, however. According to the solutions page, they're both strong!

How is $\mbox{O}^{2-}$ a strong base? How is $\mbox{OH}^{-}$ a strong acid? To my knowledge, neither are found in the canonical strong acid/base lists, and neither seem to satisfy the "completely ionize in water" definition.

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$\ce{H2O}$ is a weak acid. Abstract a proton to give $\ce{HO^{-}}$, a strong base. Abstract a proton from that to give $\ce{O^{2-}}$, a very strong base. See a species as a whole, up and down for a property.

Try it with ammonia. Feel free to wonder why $\ce{Li3N}$ is red. Look what a very strong base can do,

$\ce{Li3N + 2 H2 → LiNH2 + 2 LiH}$ (reversible at 270°C)

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  • $\begingroup$ But why is OH- a strong base? What criteria does it meet for strength? $\endgroup$ – Ennicus Pivram Mar 26 '14 at 20:27
  • $\begingroup$ Hydroxide is the strongest base allowed in water. Stronger bases give you hydroxide plus their conjugate acid. For stronger bases than hydroxide use a less acid solvent, e.g., liquid ammonia, or dipolar aprotics like DMSO, HMPA, possibly DMF or MeCN. $\endgroup$ – Uncle Al Mar 28 '14 at 18:34

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