Answer to Question #108479 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 0.00200 M solution of HCl. answer in M

Answer 1

HCl is a strong acid while fluoroacetic acid is weak acid. Strong acids on dissolution or in aqueous medium completely dissociate into protons and conjugate bases. Weak acids do not ionize completely in aqueous medium and stay in an equilibrium with unionized species always. Therefore, a solution containing 0.00200 M HCl will contain 0.00200 M concentration of protons.

pH = -log[H⁺]. So for a 0.00200 M HCl solution, pH will be -log[0.00200] = 2.69897.

pK_a is related to the dissociation constant K_a of acids as pK_a = -logK_a. The dissociation constant shows the extent to which the acid is ionized in water. This K_a is an equilibrium constant whose expression is [H⁺][A^-]/[HA] where HA is the representation of the acid where A^- is its conjugate base - the species formed upon dissociation of its protons.

Since fluoroacetic acid gives only one proton per molecule, the ratio of protons and fluoroacetate ion in the medium is in the ratio 1:1, which can be taken as x. From the K_a of the acid as K_a = 10^-pKa = 2.5704 x 10^-3, the concentration of fluoracetic acid required for 0.00200 M of protons can be given as [HA] x K_a = x². This gives the concentration of acid as 4 x 10^-6 / 2.5704 x 10^-3 = 1.56 x 10^-3 M.

Thus, a 1.56 mM (millimolar i.e. 1mM = 0.001M) solution of fluoroacetic acid will have the same pH as a 2.00 mM solution of HCl.