I know that pyrophosphite by itself is $\ce {({P_2}{O_5})^{4-}}$. My question is about the hydrogen part- how to know how many hydrogens to add? The answer key for this question says the compound is $\ce {KH_3P_2O_5}$ implying that I should add 3 hydrogens. But why not add 1 hydrogen and have $\ce {K_3}$ instead?


1 Answer 1


As phosphorous acid is biprotic:


the respective pyrophosphorous acid is biprotic too:


and pyrophosphite is:


Therefore, potassium hydrogen pyrophosphite would be:




By other words, from 4 hydrogens of pyrophosphorous acid, only 2, bound to O, are acidic. The other 2, bound directly to P, are not.

Similarly, hypophosphorous acid is monoprotic, as only 1 of 3 hydrogen atoms is acidic:


Pyrophosphite is not analogous to pyrosulphite, nor phosphite to sulfite. There is no $\ce{PO3^3-}$ nor $\ce{P2O5^4-}$.

Pyrophosphorous acid is in this context similar to acetic acid. Not all their hydrogen atoms are acidic, so even if fully neutralized, some remain. Sodium acetate is $\ce{CH3COONa}$, not $\ce{CNa3COONa}$.

  • $\begingroup$ BTW, such an acid salt is hard to achieve due to multiple equilibriums between all phosphites and hydrogenphosphite species. The closest we can have is potassium dihydrogenphosphite, $\ce{KH2PO3}$ which forms the addendum, $\ce{KH2PO3.H2(HPO3)}$. $\endgroup$ Commented Dec 25, 2022 at 13:29
  • $\begingroup$ @NilayGhosh I have quite intentionally avoided physical availability of the acid, considering formal aspect. I suspect the author of the task may have made it up to trick the resolvers, catching them on unusual attributes of acids of phosphorus. $\endgroup$
    – Poutnik
    Commented Dec 25, 2022 at 13:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.