Question

Gifblaar is a small South African shrub and one of the most poisonous plants known because it contains fluoroacetic acid (FCH2COOH), which has a pKa of 2.59. Calculate the concentration of fluoroacetic acid in a solution that has the same pH as a 0.00365 M solution of HCl.

Answer 1

HCl is strong acid and completely dissociate, so

[HCl] = [H⁺] = 0.00365M

to give pH equal to pH of 0.00365M HCl , H⁺ ion concentration of FCH2COOH solution should be 0.00365M

ionization equilibrium of FCH2COOH is

FCH2COOH <------> FCH2COO^- + H⁺  

K_a = [FCH2COO^-] [H⁺]/[FCH2COOH]

pKa = - logK_a

-logK_a = 2.59

K_a= 2.57 ×10^-3

[FCH2COO^-]= [H⁺] = 0.00365M

[FCH2COOH]_eq = [FCH2COO^-][H⁺]/K_a

[FCH2COOH]_eq = (0.00365M)²/2.57×10^-3M = 0.005184M

[FCH2COOH]eq = [FCH2COOH]_init - [FCH2COO^-]

[FCH2COOH]_init = [FCH2COOH]_eq + [FCH2COO^-]

[FCH2COOH]_init = 0.005184M + 0.00365M = 0.008834M Therefore,

Concentration of FCH2COOH in a solution that has the same pH of 0.00365M HCl solution = 0.008834M