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I couldn't come to a resolution with these 2 questions that use hydrogen in the middle of a formula that is equipped with a numerical prefix.

Why is is one inside the bracket and one outside? Am I mixing up one is a cation and the other is anion?

Aluminum monohydrogen phosphate: $\ce{Al2(HPO4)3}$

Copper(I)dihydrogen phosphate: $\ce{CuH2PO4 }$

The other question is, in hypochlorous acid (HClO), leaves out the 2 charges after the O? ex) I wrote $\ce{HClO2}$.

Do we always not mention the charges in binary acid compounds?

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    $\begingroup$ Please don't put multiple unrelated questions into one post. Also, are you confusing oxyacids with binary acids? Finally, google and Wikipedia are your friends, for instance: en.wikipedia.org/wiki/Aluminium_dihydrogenphosphate The notation reflects the composition but can also reflect the structure. $\endgroup$
    – Buck Thorn ♦
    Commented Oct 24, 2023 at 5:25
  • $\begingroup$ You can technically put the H in bracket for copper dihydrogen phosphate and it mean the same thing. It is an ionic compound comprising of cation and anion. So, by taking that into consideration, we can write $\ce{Cu^{+}(H2PO4)^{-}}$. For simplicity, remove the brackets and charge to give $\ce{CuH2PO4}$. We provide brackets to identify that the entity inside the bracket is a single anion. $\endgroup$
    – Nilay Ghosh
    Commented Oct 24, 2023 at 9:48

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$\ce{Al2(HPO4)3}$ is a compound made of $2$ ions $\ce{Al^{3+}}$ and $3$ ions $\ce{HPO4^{2-}}$. In total, this produces $6$ positive charges, and $6$ negative charges. Both number of charges are equal in the formula. Total zero. This is needed to get a neutral substance. Now, in the formula, the positive ions is at left, and the negative charges are at right. So the formula is all right. It is not absolutely forbidden, but usually it is not necessary to include the charges in the formula.

Same thing happens in the substance $\ce{CuH2PO4}$, which is made of $2$ ions, one $\ce{Cu+}$ and one $\ce{H2PO4-}$. In total, this substance contains $1$ positive ion, and $1$ negative ion. The total is zero. This is needed to get a neutral substance. Now in the formula the positive ion is written first, then the negative ion. The final formula may contain the individual charges, but usually, these charges are not necessary. So they are not part of the formula.

For the last question, you should not compare $\ce{HClO}$ and $\ce{HClO2}$ which are different substances, without any connection between them. They cannot be exchanged or compared.

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  • $\begingroup$ thank you i now well understand the use of numerical prefix in ion compounds. For the second question i forgot the difference between hydro_ acid and the existence of hypochlorite acid so the similarity between those 2 names had me confused. thank you $\endgroup$
    – Han H
    Commented Oct 25, 2023 at 4:20

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