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Topic: Henderson-Hasselbalch Equation Question  (Read 4174 times)

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Offline tangent1.57

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Henderson-Hasselbalch Equation Question
« on: July 23, 2009, 10:07:16 PM »
Given an acidic reaction:  HX --> H+  +  X-

Doesn't [H+] always equal to [X-]?

The Henderson-Hasselbalch (HH) Equation is as follows:
pH = pKa + LOG {[A-]/[HA]}. 

If we substitute [A-] for [H+], we get:

pKa = log[HA] + 2pH           However, in my organic chemistry book, this doesn't seem to be the case.  Please help me.

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Offline AWK

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Re: Henderson-Hasselbalch Equation Question
« Reply #1 on: July 24, 2009, 01:13:10 AM »
Quote
Doesn't [H+] always equal to [X-]?
but only for strong acids!
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Offline Borek

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Re: Henderson-Hasselbalch Equation Question
« Reply #2 on: July 24, 2009, 03:37:09 AM »
Quote from: tangent1.57 on July 23, 2009, 10:07:16 PM
Doesn't [H+] always equal to [X-]?

No.

Strength of acid doesn't matter.

[A-] = [H+] is a useful approximation that works if there are no other acids present, and concentration of H+ from HA is large enough so that we can ignore H+ from water autodissociation.
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