# How to write formula for potassium hydrogenpyrophosphite?

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?

As phosphorous acid is biprotic:

$$\ce{HO-PH(=O)-OH},$$

the respective pyrophosphorous acid is biprotic too:

$$\ce{HO-PH(=O)-O-PH(=O)-OH}$$

and pyrophosphite is:

$$\ce{^{-}O-PH(=O)-O-PH(=O)-O-}$$

Therefore, potassium hydrogen pyrophosphite would be:

$$\ce{KHP2H2O5}$$

respectively

$$\ce{KH3P2O5}$$

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:

$$\ce{HO-PH2=O}$$

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 $$\ce{CNa3COONa}$$.

• 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, $\ce{KH2PO3.H2(HPO3)}$. Dec 25, 2022 at 13:29
• @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. Dec 25, 2022 at 13:33