Why is monohydrogen phosphate an incorrect name?

In my Ap chem class I got this sheet with the names of a bunch of common polyatomic ions. One of them is named monohydrogen phosphate ($\ce{HPO4^2-}$). In naming, I learned not to use ‘mono-’ if it’s at the beginning of a name. 


Is this simply a mistake? Or is there a reason?




The current version of [*Nomenclature of Inorganic Chemistry – IUPAC Recommendations 2005 (Red Book)*](http://www.iupac.org/fileadmin/user_upload/databases/Red_Book_2005.pdf) reads as follows:



> 
> (…)
> 
> 
> (…) The prefix ‘mono’ is, strictly speaking, superfluous and is only needed for emphasizing stoichiometry when discussing compositionally related substances (…).
> 
> 
> (…)
> 
> 
> 


Therefore, hydrogenphosphate ($\ce{HPO4^2-}$) may be called monohydrogenphosphate in order to emphasize the difference to dihydrogenphosphate $\ce{H2PO4-}$.


Note that, according to IUPAC Recommendations, the systematic *hydrogen name* of $\ce{HPO4^2-}$ actually is ‘hydrogen(tetraoxidophosphate)(2−)’; however, ‘hydrogenphosphate’ is an *accepted simplified hydrogen name*.


 4

The reason you don't generally need "mono"at the beginning is that the charge of the ion implies how many hydrogens would be there if you simply said "hydrogen phosphate". That would be three hydrogens because phosphate has a charge of -3.


That being said, I think you are correct in saying mono-hydrogen phosphate because because "hydrogen phosphate" implies three hydrogen, not the one that you want to specify.


In the end everyone is going to write out the chemical symbols for small molecules like that so don't stress too much beyond getting those easy test questions.


 2