Question

1. Gifblaar is a small South African shrub and one of the most poisonous plants known because it contains fluoroacetic acid (FCH₂COOH), which has a pK_a of 2.59. Calculate the concentration of fluoroacetic acid in a solution that has the same pH as a 0.00745 M solution of HCl.

2. The leaves of the rhubarb plant contain high concentrations of diprotic oxalic acid (HOOCCOOH) and must be removed before the stems are used to make rhubarb pie. If pK_a1 = 1.23 and pK_a2 = 4.19, what is the pH of a 0.0327 M solution of oxalic acid?

Answer 1

1)Solution-

pKa = -logKa = 2.59
Hence, Ka = 10⁻²·⁵⁹

Let y M be the concentration of FCH₂CH₂COOH.

Being a strong acid HCl completely dissociates to give H⁺ ions.
The FCH₂CH₂COOH has the same pH as a 0.00745 M HCl.
Hence, [H⁺] in FCH₂CH₂COOH solution = 0.00745 M

The dissociation of FCH₂CH₂COOH is as follow

FCH₂CH₂COOH(aq) ⇌ FCH₂CH₂COO⁻(aq) + H⁺(aq) Ka = 10⁻²·⁵⁹
Initial: y M 0 M 0 M
Change: -0.00745 M +0.00745 M + 0.00745 M
At eqli: (y - 0.00745) M 0.00745 M 0.00745 M

At equilibrium :
Ka = [FCH₂CH₂COO⁻] [H⁺] / [FCH₂CH₂COOH]
10⁻²·⁵⁹ = 0.00745² / (y - 0.00745)
y = (0.00745^2/10⁻²·⁵⁹) + 0.00745
y = 0.029
Hnce the Concentration of FCH₂CH₂COOH = 0.029 M