What is the pH of a solution prepared by mixing equal volumes of 0.10 M aqueous hydrochloric and 0.1 M aqueous hydrofluoric acid? (K_a for HF is 6.6 × 10^-4) A)1.0 B)1.3 C)1.6 D)2.2 E)5.0

Question

Quizplus · Accepted Answer

The reaction between HCl and HF is a neutralization reaction, which can be represented as:
HCl (aq) + HF (aq) → H2O(l) + ClF(aq)
The ClF formed in the reaction is a strong acid and dissociates completely in water, producing H+ ions. Therefore, the pH of the solution will be determined by the concentration of H+ ions produced by the dissociation of HF. Using the given pKa value for HF (6.6 × 10-4), we can calculate the Ka value:
Ka = 10-pKa = 10-6.6 × 10-4 = 1.8 × 10-4
The dissociation of HF can be represented as:
HF (aq) + H2O(l) ⇋ H3O+(aq) + F-(aq)
Initially, the concentration of HF and HCl are both 0.1 M, and when they are mixed in equal volumes, each becomes 0.05 M. At equilibrium, let x be the concentration of H3O+ produced by the dissociation of HF. Then, the equilibrium concentrations of HF, F-, and H3O+ can be expressed as: 0.05 - x, x, and x, respectively. Using the Ka expression for HF, we can write:
Ka = [H3O+][F-] / [HF] = x^2 / (0.05 - x) = 1.8 × 10-4
Solving for x gives: x = 7.3 × 10-3 M
Therefore, the pH of the solution is:
pH = -log[H3O+] = -log(7.3 × 10-3) = 2.14
which is closest to choice B: 1.3.

What Is the PH of a Solution Prepared by Mixing