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What is the reason for the phosphoric acid $\ce{H3PO4}$ being a weak acid despite having a structure similar to that of sulfuric acid $\ce{H2SO4}?$

I found some sources claiming that the resonance of one oxygen with double-bonded oxygen blocks the resonance of that of $\ce{OH-},$ destabilising the negative charge. I am not sure about this reason.

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  • $\begingroup$ Dissociation of H+ ions is less favorable in H3PO4 when compared to H2SO4. One of the central P-OH groups can be in resonance with a neighboring doubly bonded oxygen. This resonance form partially shifts the negative charge away from the OH group, making it slightly less "willing" to release a proton (H+). $\endgroup$
    – Ronith
    Commented May 7 at 15:58
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    $\begingroup$ About the reason you gave about cross conjugation , it is also present in sulphuric acid. In sulphuric acid it is only from two sides but that is not the main reason why phosphoric acid is weak and sulphuric acid is strong. $\endgroup$ Commented Jun 6 at 18:27
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    $\begingroup$ Not that it's all that weak, at least in the first dissociation. $\endgroup$ Commented Jun 6 at 20:36

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There are few reasons why $\ce{H3PO4}$ is much weaker acid than $\ce{H2SO4}$:

  • $\ce{P}$ has much lower electronegativity than $\ce{S}$, leading to making a weaker acid.
  • One $\ce{O}$ atom less, that
    • makes the acid stronger by decreasing electron density on $\ce{P}$ atom.
    • makes the conjugate base weaker by charge delocalization.
  • One $\ce{OH}$ group more, that makes the acid weaker by increasing electron density.
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