For H3PO4, pK_a2 = 7.21 and pK_a3 = 12.35. Treating HPO4^2- as an amphiprotic species with pH = tfrac12(pK_a2+pK_a3), find the pH of an aqueous Na2HPO4 solution (two decimals).

JEE Main levelNumerical answerDifficulty 2 of 3ia179

For $H X_{3} PO X_{4}$ , $p K_{a 2} = 7.21$ and $p K_{a 3} = 12.35$ . Treating $HPO X_{4} X^{2 -}$ as an amphiprotic species with $pH = \frac{1}{2} (p K_{a 2} + p K_{a 3})$ , find the pH of an aqueous $Na X_{2} HPO X_{4}$ solution (two decimals).

Numerical answer type, tolerance 0.01.

HPO X_{4} X^{2 -}

is the intermediate between

H X_{2} PO X_{4} X^{-}

and

PO X_{4} X^{3 -}

, so its pH is the mean of the two flanking constants:

(7.21 + 12.35) /2 = 9.78