Hydrofluoric acid solution?

A solution of hydrofluoric acid contains 2.0g of HF per litre and has a pH of 2.2. What is the dissociation constant for HF?

Answer:
pH = - log [H+]
[H+] = 10^- 2.2 = 0.0063 M

molar mass HF = 21 g / mol
2.0 g / 21= 0.095 moles HF
initial concentration HF = 0.095 / 1 L = 0.095 M

HF <-------> H+ + F-
at equilibrium
0.095-0.0063..0.0063. 0.0063

Ka = 0.0063 x 0.0063 / 0.0889 = 4.5 x 10^-4
first you need to find the H+ concentration in the solution, so take 10^-2.2 to find that (reverse logarithm). this will also be the F- concentration.

Then, find the molarity of the original HF solution before stuff started to dissociate by finding the moles of HF in 2.0g.

Last, multiply the H+ concentration by the F- concentration and divide by the HF concentration. this will be the Ka of HF.

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