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 0.00770 M solution of HCl

Answer 1

[H_{3}O^{+}] = 0.00770 M

The equilibrium equation representing the dissociation of $FCH_{2}COOH$

$FCH_{2}COOH(aq) + H_{2}O (l) FCH_{2}COO^{-}(aq)+ H_{3}O^{+}(aq)$

Given [H_{3}O^{+}] = 0.00770 M

Let the initial concentration of acid be x and change y

So y = $[H_{3}O^{+}]$ =$[FCH_{2}COO^{-}]$ = 0.00770 M

$pK_{a} = 2.59K_{a} = 10^{-2.59} = 0.00257 M$

$K_{a} = \frac{(0.00770 M)(0.00770 M)}{x - 0.00770}$

$0.00257 = \frac{0.00005929}{x - 0.00770}$

0.00257 x - 0.00001979 = 0.00005929

x = 0.031 M

Therefore, initial concentration of the weak acid is 0.031 M