I am a chemistry expert with a deep understanding of the principles that govern the strength of acids. Let's delve into the factors that determine why H3PO4 (phosphoric acid) is a stronger acid than H3AsO4 (arsenic acid).

The strength of an acid is determined by its ability to donate protons (H+ ions) in solution. A stronger acid is one that donates protons more readily. There are several factors that influence the strength of an acid, including the electronegativity of the central atom, the bond strength between the central atom and the hydrogen atoms, the size of the central atom, and the resonance stabilization of the conjugate base.

Electronegativity plays a crucial role in acid strength. The more electronegative an atom is, the more it attracts the electrons towards itself, which in turn weakens the O-H bond and makes it easier for the acid to donate a proton. Phosphorus (P) is less electronegative than chlorine (Cl), but more electronegative than arsenic (As). This means that in H3PO4, the O-H bonds are more polarized than in H3AsO4, making it easier for H3PO4 to lose a proton.

Bond Strength is another factor. The strength of the O-H bond affects how easily an acid can donate a proton. A weaker bond means the acid can more readily donate a proton. Phosphorus forms weaker O-H bonds compared to arsenic due to its smaller size and higher electronegativity, which facilitates the release of protons in H3PO4.

The Size of the Central Atom also influences acid strength. A larger central atom can lead to a less effective overlap of the orbitals, which can result in weaker O-H bonds. Arsenic is larger than phosphorus, which would generally suggest that H3AsO4 should be a stronger acid. However, this is counteracted by the higher electronegativity of phosphorus and the resonance stabilization in H3PO4.

Resonance Stabilization is a significant factor in the case of H3PO4. When H3PO4 loses a proton, it forms H2PO4-, which can resonate, distributing the negative charge over three oxygen atoms. This resonance stabilization makes H2PO4- a more stable conjugate base, which in turn makes H3PO4 a stronger acid. In contrast, the conjugate base of H3AsO4, H2AsO4-, does not have the same level of resonance stabilization, making it a weaker acid.

It's also important to note that the number of oxygen atoms surrounding the central atom can affect the acid strength. More oxygen atoms can lead to better stabilization of the negative charge after deprotonation, enhancing the acid's strength.
H3PO4 has three oxygen atoms surrounding the phosphorus atom, which contributes to its strength.

In conclusion, while the size of the central atom might suggest that H3AsO4 should be a stronger acid, the combined effects of higher electronegativity, weaker O-H bonds, and resonance stabilization in H3PO4 make it the stronger acid.

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HCl is a stronger acid than H2S because Cl is more electronegative than S. H2AsO4- is a stronger acid than HAsO42- because it has more acidic H atoms. H3AsO4 is a stronger acid than H3PO4 because As is larger than P. HBrO3 is a stronger acid than HBrO2 because it has more oxygens surrounding the central Br atom.read more >>