Hydrolysis of salts of weak acid-weak base


by Rishavutkarsh
Tags: acidweak, base, hydrolysis, salts, weak
Rishavutkarsh
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#1
Dec21-13, 02:46 AM
P: 56
I just came across the hydrolysis of the salts and the formula
pH=7+1/2(pKa-pKb)
What's bugging me about this is how can this be independent of Concentration?
I mean if we dilute it why wont it affect the pH? That means that even a negligible amount of such a salt can affect the pH of a bucket of water? Also there is no assumption involved so please explain how exactly is this plausible, thanks.
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Borek
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#2
Dec21-13, 03:43 AM
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It is only approximate, and works reasonable well as well as the salt concentration is reasonably high.

See the derivation and discussion on these pages:

http://www.chembuddy.com/?left=pH-ca...alt-simplified

http://www.chembuddy.com/?left=pH-ca...phiprotic-salt

(the latter is about slightly different case, but the conclusions about concentrations are very similar).


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