What factor primarily determines the acid strength of an oxo-acid?

The strength of an oxo-acid is primarily determined by the stability of its conjugate base. When an oxo-acid donates a proton (H+), it forms a conjugate base that can play a significant role in defining the overall acidity of the compound. A more stable conjugate base means that the ion is less likely to revert back to the original acid, leading to higher acidity.

Factors that contribute to the stability of the conjugate base include resonance, where the negative charge can be delocalized over several atoms, and inductive effects from electronegative substituents that can help stabilize the negative charge. If the conjugate base can effectively redistribute or stabilize the charge, the acid itself will be stronger.

While electronegativity of the central metal and the size of the metal ion can play roles in influencing the acidity, they are often secondary factors in comparison to the stability of the conjugate base. The number of hydroxyl groups can also affect acidity, as more hydroxyls can provide more sites for protonation, but the central factor remains the stability of the formed conjugate base after proton release.