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Chemistry
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Hydrofluoric acid weak monoprotic acid with Ka = 6.6×10-4 M. 

1)Calculate pH  6.99×10-2 M solution of sodium fluorine

Feb 16th, 2015

Kb = Kw / Ka = (10-14) / (6.6*10-4) = 1.5*10-11

F-  +  H2O  ó  HF  +  OH-

Kb = (1.5*10-11) = x2 / (0.0699 – x )  [rearrange and quadratic formula]

Gives x = 1.02*10-6 M = [OH-]

So pOH = -log(1.02*10-6) = 6.0

And pH + pOH = 14  =>  pH = 14 – 6.0 = 8.0 

Feb 16th, 2015

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