Chapter 16, Problem 108GQ

The hydrogen phthalate ion, C₈H_sO₄⁻, is a weak acid with K_a = 3.91 × 10⁻⁶.

${\text{C}}_8{\text{H}}_5{\text{O}}_4^{-}{\text{(aq) + H}}_{\text{2}}\text{O(}\mathcal{l}\text{) }\rightleftarrows {\text{ C}}_8{\text{H}}_4{\text{O}}_4^{2-}{\text{(aq) + H}}_3{\text{O}}^{+}\text{(aq)}$

What is the pH of a 0.050 M solution of potassium hydrogen phthalate. KC₈H₅O₄? Note: To find the pH for a solution of the anion, we must take into account that the ion is amphiprotic. It can be shown that, for most cases of amphiprotic ions, the H₃O⁺ concentration is

${\text{[H}}_3{\text{O}}^{+}\text{] = }\sqrt{K_{\text{a1}}\times K_{\text{a2}}}$

For phthalic acid, C₈H₆O₄ is K_a1 is 1.12 × 10⁻³, and K_a2 is 3.91 × 10⁻⁶.