I have seen two reactions of $\ce{H2SO4}$, in one it reacts with $\ce{P2O5}$ and in the other $\ce{P4O10}$.

The Reactions are as follows:

$$\ce{H2SO4 + P2O5 -> SO3 + 2HPO3}$$

$$\ce{6H2SO4 + P4O10 -> 6SO3 + 4H3PO4 }$$

Since it is a dehydration reaction the oxidation states of sulphur and phosphorous would be the same in reactant and product, this is true for both the reactions.

Are the both Products correct $\ce{HPO3}$ and $\ce{H3PO4}$ correct? If yes what I am missing?

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    $\begingroup$ P2O5 and P4O10 is the same thing. $\endgroup$ Jul 3, 2021 at 12:37
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    $\begingroup$ I agree with Ivan Neretin. Basically, $\ce{P4O_{10}}$ is the dimeric form of $\ce{P2O_{5}}$. $\endgroup$ Jul 3, 2021 at 12:46
  • 1
    $\begingroup$ To clarify, read here. $\endgroup$ Jul 3, 2021 at 12:51
  • $\begingroup$ Yes @IvanNeretin ... I agree with it but why two different products . Then may be one of them is incorrect then which one ? $\endgroup$ Jul 3, 2021 at 12:52
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    $\begingroup$ @MathewMahindaratne I object. P4O10 is the only form of P2O5 (not counting the polymeric forms, that is). There is no monomer of which it is a dimer. $\endgroup$ Jul 3, 2021 at 16:16

1 Answer 1


Phosphorus pentoxide is the common name of a chemical with molecular formula $\ce{P4O10}$. The common name is derived from its empirical formula, $\ce{P2O5}$. This white crystalline solid is the anhydride of phosphoric acid:

Phosphorus pentoxide

It is a potent dehydrating agent and used as a powerful desiccant (as indicated by the exothermic nature of its hydrolysis) in limited capacity: $$\ce{P4O10 + 6H2O -> 4H3PO4} \qquad \Delta H = \pu{-177 kJ}$$

However, the desiccating power of $\ce{P4O10}$ is strong enough to convert many mineral acids to their anhydrides. For examples, $\ce{H2SO4}$ is converted to $\ce{SO3}$: $$\ce{P4O10 + 6H2SO4 -> 6SO3 + 4H3PO4} $$

Therefore, it can be concluded that the second reaction is the correct one (although it may or may not be balanced). The first one is incorrect, solely based on the fact that $\ce{P2O5}$ does not exist.

  • 1
    $\begingroup$ Hm, I have never seen P4O10 during describing dyhydration reactions to produce H3PO4. Especially when dehydrating other dehydrating agens. I usually see written the product as polymetaphosphoric acid (HPO3)n $\endgroup$
    – Poutnik
    Jul 4, 2021 at 6:18

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