Question

The ionization constant (Kᵃ) of HF is 6.7 × 10⁻⁴. In a 0.1M solution of this acid, which of the following is true? a) [H⁺] > [F⁻]

b) [H⁺] = [F⁻]
c) [H⁺] < [F⁻]
d) Kᵃ = [H⁺][F⁻]

Answer 1

In a 0.1M solution of HF, the concentration of H+ is equal to that of F- due to the stoichiometry of its dissociation, even though the Kᵅ suggests not all HF will dissociate. The statement [H+] = [F-] is true for this scenario. The correct answer is option (b).

Step-by-step explanation:

The ionization constant (Kᵅ) of HF is 6.7 × 10⁻⁴. For a 0.1M solution of this acid, we must consider the degree to which the acid dissociates in water. Given the Kᵅ value, we know that not all HF will dissociate into H+ and F− ions. In this scenario, the concentration of H+ will be equal to the concentration of F− due to the stoichiometry of the dissociation reaction, resulting in b) [H+] = [F−].

Choice d) Kᵅ = [H+][F−] is a misstatement; the correct expression is Kᵅ = [H+][F−]/[HF]. Given the small value of Kᵅ, we can assume that the dissociation of HF will be suppressed upon adding F−, as per Le Châtelier's principle, which shifts the equilibrium to favor the reactants, resulting in a negligible change in F− concentration.