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The dissociation of HF follows this equation:
HF <------> H+ + F- Ka = 7.2 * 10^(-4) = .00072
By the definition of Ka, this means that:
[H+]*[F-] / [HF] = .00072
where [x] means concentration of x.
You can use the pH, which is given to be 4.4, to figure out the concentration of H+. If the pH is 4.4, the concentration of H+ is always 10^(-4.4).
Plug that in to the first equation:
[10^-4.4]*[F-] / [HF] = .00072
And you get:
[F-] / [HF] = 18.1
Another way to do this is by using the Henderson-Hasselbach equation, which says that:
pH = pKa + log([A-]/[HA])
If you use the pKa of HF and set F- as A- and HF as HA, you'll get the same answer for [F-]/[HF].
Now that you know the ratio between the HF and F-, you can easily figure out the percentage:
18.1/19.1 = 94.7% F-
1/19.1 = 5.3% HF
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