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Topic: Henderson Hasselbalch Equation only for Weak acids?  (Read 3617 times)

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Offline SinkingTako

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Henderson Hasselbalch Equation only for Weak acids?
« on: November 09, 2012, 07:08:05 AM »
Is Henderson Hasselbalch Equation only suitable for weak acids?

I have a question as follows:

0.5 mol of HA (Ka = 1.5×10-3) was added to 3.0 L of water. After equilibrium was reached, 1.0 g of NaOH was added to the solution. What is the new [H+]?

Okay, so if I find that there is 0.025mol of NaOH, then [NaA] formed is 8.3333 x 10^-3M

As for HA, the new concentration after NaOH reaction is 0.15833M.

Assuming that NaA will dissociate fully, [A-] will be 8.3333 x 10^-3M. If I now just use the
Henderson Hasselbalch Equation ([salt] is 8.333 x 10-3, [acid]=0.15833), the pH of the final solution will be 1.54. However, looking at the original solution, the pH is 1.82 ???

If I use the ICE table, do brute force using Ka, then the pH I obtain is 1.95, which is still reasonable... 

Thus, what is wrong with this calculation? Or is it that in this case, in order to use the equation, the [salt] is the sum of [A-] from both the NaA formed and the dissociation of HA?

Thanks in advance!
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Offline Borek

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Re: Henderson Hasselbalch Equation only for Weak acids?
« Reply #1 on: November 09, 2012, 12:05:58 PM »
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Offline SinkingTako

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