For H3PO4, pK_a1 = 2.15, pK_a2 = 7.21, pK_a3 = 12.35. Using pH = tfrac12(pK_before+pK_after) for amphiprotic ions, select the correct statement(s).

At pH = 7.21, [H2PO4-] = [HPO4^2-]

A NaH2PO4 solution has pH approx 4.68

A Na2HPO4 solution has pH approx 9.78

At pH = 7.21, [H2PO4-] = [HPO4^2-]

A NaH2PO4 solution has pH approx 4.68

A Na2HPO4 solution has pH approx 9.78

At pH = 2.15, [H3PO4] = [HPO4^2-]

JEE Main levelOne or more correct optionsDifficulty 2 of 3ia180

For $H X_{3} PO X_{4}$ , $p K_{a 1} = 2.15$ , $p K_{a 2} = 7.21$ , $p K_{a 3} = 12.35$ . Using $pH = \frac{1}{2} (p K_{before} + p K_{after})$ for amphiprotic ions, select the correct statement(s).

Mean-of-pKa rule gives

NaH X_{2} PO X_{4}

4.68

and

Na X_{2} HPO X_{4}

9.78

. When

pH = p K_{a 2}

[H X_{2} PO X_{4} X^{-}] = [HPO X_{4} X^{2 -}]

. When

pH = p K_{a 1}

, it is

[H X_{3} PO X_{4}] = [H X_{2} PO X_{4} X^{-}]

, and

[HPO X_{4} X^{2 -}]

is far smaller, so the last statement is false.