Is H2PO4- stronger than HCN? [closed]








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Wouldn't $\ce{HCN}$ be stronger, since $\ce{H2PO4-}$ predominantly acts as a base in aqueous solution?




As Zhe points out, dihydrogen phosphate ion [is not predominantly basic in water](http://clas.sa.ucsb.edu/staff/Resource%20folder/Chem109ABC/Acid,%20Base%20Strength/Table%20of%20Acids%20w%20Kas%20and%20pKas.pdf).


But the larger answer is that one thing can be both a stronger acid and a stronger base than another. For example, zinc hydroxide reacts with both acids and bases in water whereas carbon monoxide reacts with neither (at least under normal laboratory conditions). So zinc hydroxide is both a stronger acid and a stronger base than carbon monoxide.


Similarly any basicity in the dihydrogen phosphate ion does not rule out the possibility that it could also be an acid. But there is a limit to this, things that are both acidic and basic, called **amphoteric**, are generally weak in both directions.


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