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Home  FAQ Acids and bases | Print | Comment | Contact |
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How is pKa related to pKb?
- How can I find a relationship to pKa and pKb using pK = - log K ?
Alex Baskin baskin@scruznet.com -
Imagine you have an acid dissociation
followed by hydrolysis of the A- ion,HA 
H+ + A-
Ka = [H+][A-]
[HA]
If you add the two reactions together, the net reaction is the hydrolysis of water,A- + H2O HA + OH-
Kb = [OH-][HA]
[A-]
Looking at the equilibrium expressions, you can see that the product of Ka and Kb is Kw:H2O H+ + OH-
Kw = [H+][OH-] = 1.01×10-14
Take minus the log of both sides of this equation and you get pKa + pKb = pKw.Ka Kb = [H+][A-]
[HA]× [OH-][HA]
[A-]= [H+][OH-] = Kw
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