Answer to Question #331755 in General Chemistry for MBM

Question

Calculate the initial concentration of Fluoroacetic acid in a solution that has the same pH as a 0.00865 M solution of HCl

Answer 1

HCl is a strong acid and dissociates completely to H⁺ and Cl^-, therefore

pH = -lg[H⁺] = -lgC(HCl) = -lg(0.00865) = 2.063

The equilibrium of dissociation is FCH₂COOH = FCH₂COO^- + H⁺

the equilibrium constant is K_a = [H⁺]*[FCH₂COO-]/[FCH₂COOH]

C is initial concentration of the acid,

equilibrium concentrations are equal for [H⁺] = [FCH₂COO^-] = 10^-2.063 = 0.00865 M,

equilibrium concentration of the acid is [FCH₂COOH] = C - [H⁺] = C - 0.00865 M, hence:

K_a = [H⁺]²/(C-[H⁺])

10^-2.59 = 0.00257 = (0.00865)²/(C-0.00865),

C = (0.00865)²/0.00257 + 0.00865 = 0.00378 M

The initial concentration is 3.78*10^-3 M