Why is potassium phosphate KH2PO4 in this reaction? This is a continuation of this question because the first thing that came in my mind is that why potassium phosphate in this reaction is $\ce{KH2PO4}$ and not $\ce{K3PO4}$?

In the wikipedia article of phosphate ion ($\ce{PO4^3-}$), it is written that:


  Aqueous phosphate exists in four forms. In strongly basic conditions,
  the phosphate ion ($\ce{PO4^3-}$) predominates, whereas in weakly
  basic conditions, the hydrogen phosphate ion ($\ce{HPO4^2-}$) is
  prevalent. In weakly acidic conditions, the dihydrogen phosphate ion
  ($\ce{H2PO4−}$) is most common. In strongly acidic conditions,
  trihydrogen phosphate ($\ce{H3PO4}$) is the main form.


So, $\ce{FeCl2}$ must somehow be acidic in aqueous solution so as to protonate potassium phosphate $\ce{K3PO4}$ to $\ce{KH2PO4}$. Is this statement true? This question throws some light on acidity of chromium chloride in aqueous solution but is it applicable to iron chloride as well?



In the previous question, the homework question was write the equation of reaction of Iron(II) chloride and potassium phosphate. So, students can assume the reaction to be $\ce{FeCl2 + K3PO4}$ to simply yield iron(III) phosphate or iron(II) phosphate and potassium chloride but why make the reaction complex by assuming potassium phosphate $\ce{KH2PO4}$.

Iron hydrogen phosphate are new in this world and has been recently synthesised and as not as stable as iron phosphates. So, can we just assume that reaction of Iron(II) chloride and potassium phosphate to yield iron (II or III) phosphate and potassium chloride?
I think that’s just another mistake the OP of the other question made. Potassium phosphate is $\ce{K3PO4}$ and any student using any hydrogenphosphate instead would lose marks if I were marking the exam.

If $\ce{KH2PO4}$ is meant to be added, use the correct term: potassium dihydrogenphosphate.

This nomenclature can be found in the current version of IUPAC’s Red Book, sections IR-8.2 and IR-8.4 and the anions in question are explicitly named in table IR-8.1 therein.