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Write the reaction for a a sodium borate solution being titrated by hydrobromic acid, assuming sodium borate is represented by $\ce{Na3BO3(aq)}$.

I think this is a simple double replacement reaction:

$$\ce{Na3BO3(aq) + 3 HBr(aq) -> 3 NaBr(aq) + H3BO3(aq)}\tag{R1}$$

Official solution without explanation:

$$\ce{Na3BO3(aq) + HBr(aq) -> 2 NaBr(aq) + NaH2BO3(aq) }\tag{R2}$$

I don't understand how the official reaction was reached. Why should a sodium cation take the place of a hydrogen?

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    $\begingroup$ It is rather $\ce{Na[B(OH)4]}$ $\endgroup$
    – Poutnik
    Nov 25 at 6:53
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    $\begingroup$ The whole task seems to me like being made up. Who would use HBr for titration and end titration with alkalic solution ? I also do not believe much in Na3BO3(aq). $\endgroup$
    – Poutnik
    Nov 25 at 7:39
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    $\begingroup$ Don't forget, in contrast to e.g., $\ce{S}$ in $\ce{H2SO4}$, $\ce{B}$ in $\ce{H3BO3}$ (or $\ce{B(OH)3}$) still has the electron gap characteristic for Lewis acids like $\ce{AlCl3}$. $\endgroup$
    – Buttonwood
    Nov 25 at 7:40
  • $\begingroup$ Poutnik is right. $\ce{Na3BO3}$ does not exist. $\endgroup$
    – Maurice
    Nov 25 at 21:46
  • $\begingroup$ @Maurice Googling "trisodium orthoborate, na3bo3" give me 168 hits. The promising results are this and this. $\endgroup$ Nov 26 at 5:39
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The borate anions are a series whose charge depends upon pH. A good (and long, but probably still incomplete) reference is https://europepmc.org/article/pmc/6470676.

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The request for an equation identifies two starting materials but does not suggest an endpoint, and there are many. The OP gives a complete reaction: react all the possible basic anions with acidic HBr. Good answer. The official solution is a partial neutralization - to what pH? The suggestion of titration needs to say what indicator or what pH is the endpoint, or else it must have been explained in class beforehand.

Or, the question is just a bad one - or a trap.

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  • $\begingroup$ Wikipedia says at boric acid that Polyborate anions are formed at pH 7–10 if the boron concentration is higher than about 0.025 mol/L. The best known of these is the 'tetraborate' ion, found in the mineral borax. $\endgroup$
    – Poutnik
    Nov 25 at 18:16
  • $\begingroup$ @James Gaidis. Where have you found these diagrams of Boron stuff versus pH ? $\endgroup$
    – Maurice
    Nov 25 at 21:41
  • $\begingroup$ @Maurice: the reference for the diagram is in the third line of my answer. $\endgroup$ Nov 26 at 16:41

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