What is the predominant form of H3PO4 in solution at pH 5?

At a pH of 5, we need to determine the predominant species of phosphoric acid (H₃PO₄) in solution based on its dissociation constants (K_a values).

Phosphoric acid is a triprotic acid, meaning it can lose three protons. The dissociation reactions are as follows:

1. First dissociation: H₃PO₄ <=> H₂PO₄^– + H⁺ (K_a1 = 7.1 x 10^-3)
2. Second dissociation: H₂PO₄^– <=> HPO₄^2- + H⁺ (K_a2 = 6.3 x 10^-8)
3. Third dissociation: HPO₄^2- <=> PO₄^3- + H⁺ (K_a3 = 4.5 x 10^-13)

To find out the predominant species at pH 5, let’s analyze the situation:

• At pH 5, the concentration of H⁺ ions in the solution is approximately 10^-5 M.
• The first dissociation, with K_a1 = 7.1 x 10^-3, indicates that H₃PO₄ dissociates significantly at this pH, meaning that a considerable amount of H₂PO₄^– will form.
• The second dissociation (K_a2) at pH 5 is only partially favored because K_a2 = 6.3 x 10^-8 is much smaller than 10^-5, meaning H₂PO₄^– will not fully convert into HPO₄^2-.
• The third dissociation (K_a3 = 4.5 x 10^-13) is negligible at this pH, so PO₄^3- will not be present in significant amounts.

Thus, with the given pH, the predominant form of H₃PO₄ in the solution will be H₂PO₄^– while H₃PO₄ and HPO₄^2- will also be present but in lesser amounts. Therefore, at pH 5, the predominant form of H₃PO₄ in solution is H₂PO₄^–.