Answer to Question #108478 in General Chemistry for Beverlie

Question

1. Gifblaar is a small South African shrub and one of the most poisonous plants knownbecauseit contains fluoroacetic acid (FCH2COOH), which has a pKaof 2.59.
Calculate the initial concentration of fluoroacetic acid in a solution that has the same pH as a 0.00135 M solution of HCl. answer in M

Answer 1

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

pH = -lg[H⁺] = -lgC(HCl) = -lg0.00135 = 2.87

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

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

C is initial concentration of the acid,

equilibrium concentrations are equal for [H⁺] = [FCH₂COO^-] = 10^-2.87 = 0.00135 M,

equilibrium concentration of the acid is [FCH₂COOH] = C - [H⁺] = C - 0.00135 M, hence:

Ka = [H⁺]²/(C-[H⁺])

10(-2.59) = 0.00257 = (0.00135)²/(C-0.00135),

C = (0.00135)²/0.00257 + 0.00135 = 0.00206 M

The initial concentration is 2.06*10^-3 M