A phosphate buffer has pH 7.60. For the pair H2PO4-/HPO4^2-, pK_a2 = 7.20. Calculate the ratio [HPO4^2-]/[H2PO4-] to two decimal places.

JEE Main levelNumerical answerDifficulty 2 of 3ia029

A phosphate buffer has pH 7.60. For the pair $H X_{2} PO X_{4} X^{-} / HPO X_{4} X^{2 -}$ , $p K_{a 2} = 7.20$ . Calculate the ratio $[HPO X_{4} X^{2 -}] / [H X_{2} PO X_{4} X^{-}]$ to two decimal places.

Numerical answer type, tolerance 0.02.

pH = p K_{a 2} + lo g ([HPO X_{4} X^{2 -}] / [H X_{2} PO X_{4} X^{-}])

, so the ratio

= 1 0^{7.60 - 7.20} = 1 0^{0.40} = 2.51