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 0.00520 M solution of HCl.

Answer 1

0.00520 M solution of HCl

HCl is a strong acid and will dissociate completely.

So, [HCl] = [H3O⁺] = 0.00520 M

pH = - log [H3O⁺]

= - log(0.00520)

= 2.28

Now, fluoroacetic acid in a solution that has the same pH as a 0.00520 M solution of HCl.

So, pH of fluoroacetic acid = 2.28

[H3O⁺] in fluoroacetic acid = 0.00520 M

Let the  fluoroacetic acid is represented as HA.

Make ICE table.

            HA     +      H2O      ---->      A-      +     H3O+
IC: C                                          0                  0
C:         -x                                          +x                +x
EC: C - x                                        x                   x

Ka = [A^-] [H₃O⁺] / [HA]

Ka = (x) (x) / (C – x)

Since [H3O+] = 0.00520 M

So, x = 0.00520 M

pKa = 2.59

- log Ka = 2.59

Ka = 10^-2.59 = 0.00257

Now,

0.00257 = (0.00520) (0.00520) / (C – 0.00520)

(C – 0.00520) = (0.00520) (0.00520) / (0.00257)

(C – 0.00520) = 0.010521

C = 0.010521 + 0.00520

C = 0.015721 M

Hence, the concentration of fluoroacetic acid in a solution is 0.015721 M