Which is the most prevalent species in a 0.25 m aqueous solution of phosphoric acid h3po4 g?

Answer is: H₃PO₄.

A phosphoric acid is three protic acid, which means that in water release tree protons. Phosphoric acid ionizes in three steps in water.

First step: H ₃PO₄ (aq) ⇄ H₂PO₄⁻ (aq) + H⁺ (aq).

Second step: H ₂PO₄⁻ (aq) ⇄ HPO₄²⁻ (aq) + H⁺ (aq).

Third step: HPO ₄²⁻ (aq) ⇄ PO₄³⁻ (aq) + H⁺ (aq).

Species that are present: H ₃PO₄, H₂PO₄⁻, HPO₄²⁻, PO₄³⁻ and H⁺, but because the acid dissociation constants are low, very low percentage of acid dissociates.