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:


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




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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 $\ce{CNa3COONa}$.


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