Determine the value of Ka for phenol.

Question

A solution of phenol (HC₆H₅O) is prepared by dissolving 0.385 g of phenol in 2.00 L of H₂O. The solution has a pH = 6.29. Determine the value of Ka for phenol.

Answer 1

From pH, determine [H⁺]:

pH = 6.29 and [H⁺] = 10^-6.29

[H⁺] = 5.13x10^-7

HC6H5O ==> H⁺ + C6H5O^-

Ka = [H⁺][C6H5O^-] / [HC6H5O]

[HC6H5O] = 0.385 g x 1 mol/94.1 g = 0.00409 moles/2.0 L = 2.05x10^-3 M

Ka = (5.13x10^-7)² / 2.05x10^-3

Ka = 1.28x10^-10