The first disinfectant used by Joseph Lister was called carbolic acid. This substance now is known as phenol, C₆H₅OH (pK_a = 10.0). What is the pH of a 0.10 M solution of phenol?

Question

Quizplus · Accepted Answer

The pKa of phenol is 10.0, which means at pH 10.0, half of the phenol molecules will be ionized and half will be in their protonated form. Therefore, at a pH lower than 10.0, phenol will be mostly in its protonated form. The formula for the acid dissociation constant (Ka) is Ka = [H+][A-]/[HA]. At equilibrium, the concentration of [A-] equals the concentration of [H+], so Ka = [H+]^2/[HA]. Since we are dealing with a weak acid, we can assume that [HA] = [phenol]. Therefore, Ka = [H+]^2/0.10 M. Rearranging the equation to solve for [H+], we get [H+] = sqrt(Ka x [phenol]) = sqrt(10^-10 x 0.10) = 10^-5 M. Taking the negative log of this value gives us a pH of 5. Therefore, the correct answer is (D) 5.5.

The First Disinfectant Used by Joseph Lister Was Called Carbolic