Question

If a solution of HF (Ka=6.8×10^−4) has a pH of 3.50, calculate the concentration of hydrofluoric acid.

So far, I've made 5 incorrect attempts at the answer and have one attempt remaining. My answers were as follows:

0.00015, 1.5x10^-4, 1.5x10^-5, 0.001, and 0.00032 (This last answer prompted the website to tell me: "This value is the concentration of H+. This concentration and the Ka value can be used along with the equation for Ka to determine the initial concentration of hydrofluoric acid. Ka=[H+][F−] / [HF]").

Answer 1

HF is a weak acid x_a = [H^+][F^-]/[HF] = 6.8 times 10^- 4 p^4 = 3.50 because [H^+] = 10^- 3.5 hence [F^0] = 10^- 3.5 let the concentration of [HF] = x because k_a = 10^- 35 times 10^- 3.5/x - 10^- 3.5 = 6.8 times 10^- 4 x - 10^- 3.5 = (10^- 3.5) ^2/6.8 times 10^- 4 x - 10^- 3.5 = 1.47 times 10^- 4 x = 1.47 times 10^- 4 + 10^- 3.5 = 4.63 times 10^- 4