If a solution of HF (Ka = 6.8 × 10⁻⁴) has a pH of 2.50, what is the initial concentration of hydrofluoric acid in the solution?

• A. 1.0 M
• B. 0.10 M
• C. 0.001 M
• D. 0.01 M

Answer: (B)

Verified step by step guidance

1. Start by understanding that HF is a weak acid, and its dissociation in water can be represented by the equation: HF ⇌ H⁺ + F⁻.
2. Use the given pH to find the concentration of H⁺ ions in the solution. Recall that pH = -log[H⁺], so [H⁺] = 10^(-pH). Substitute the given pH value to find [H⁺].
3. Set up the expression for the acid dissociation constant (Ka) for HF: Ka = [H⁺][F⁻]/[HF]. Since HF is a weak acid, assume that the initial concentration of HF is approximately equal to the concentration of HF at equilibrium.
4. Assume that the concentration of H⁺ ions is equal to the concentration of F⁻ ions at equilibrium, as they are produced in a 1:1 ratio from the dissociation of HF.
5. Rearrange the Ka expression to solve for the initial concentration of HF: [HF] = [H⁺]²/Ka. Substitute the values for [H⁺] and Ka to find the initial concentration of HF.