Oxoacids and acid anhydrides

Oxoacids are acids that donate protons in aqueous solution that previously wre bonded to oxygen atoms. Examples include sulfuric acid H$_2$SO$_4$, nitric acid HNO$_3$, phosphoric acid H$_3$PO$_4$, and carbonic acid H$_2$CO$_3$.

An acid anhydride is formed by removing enough water from an oxoacid that only the binary oxide remains. Examples follow below:

1.

Starting with carbonic acid, we remove one ${\rm H}_2{\rm O}$ unit, which leaves the oxide carbonic dioxide CO$_2$, the anhydride of carbonic acid.

2.

Phosphoric acid H$_3$PO$_4$ has an odd number of hydrogens, which means if we take away one water, we still have a hydrogen remaining. But if we make a dimer, H$_6$P$_2$O$_8$, then we can remove 3 waters, leaving the anhydride P$_2$O$_5$, which is a correct empirical formula for the actual anhydride P$_4$O$_{10}$.

Oxides of most nonmetals are acid anhydrides, which react with an excess of water to form an acidic solution. For example,

$${\rm N}_2{\rm O}_5(s) + {\rm H}_2{\rm O}(l)\longrightarrow 2... ..._3(aq)\longrightarrow 2{\rm H}^+(aq) + 2{\rm N}{\rm O}_3^-(aq)$$

Acid anhydrides are not BL acids, since they do not have protons to donate, however, they are Lewis acids, since they can accept lone pairs.