Please please help!!! I have been trying to figure this question out for 2 days and I can’t seem to do it. The solution is more important than the answer for me and so please explain and show your work when answering! Thank you sooo much !
1 Answer

HF —> H+ + F
Ka = [H+][F] / [HF] = 7.2 x 10^4
Let s = amount of HF that dissociates (ionizes)
[H+] = s
[F] = s
[HF] = 0.020 – s
7.2 x 10^4 = (s)(s) / (0.020 – s)
s^2 = (0.020 – s) x 7.2 x 10^4
s^2 = 0.0000144 – 0.00072 s
s^2 + 0.00072 s – 0.0000144 = 0
Solve the quadratic for s. A quadratic equation solver is found at:
math.about.com/library/blquadraticcalc.htm
The roots are: 0.0034517712418244616 and (0.004171771241824462)
The only answer that makes physical sense is: s = 0.00345
[H+] = [F] = 0.00345 M
[HF] = 0.020 – 0.00345 = 0.017 M