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Topic: Preparing Buffer Solutions  (Read 3642 times)

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Offline Link The Worlds

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Preparing Buffer Solutions
« on: September 16, 2010, 08:10:19 PM »
I'm really lost for this assignment and don't even know where to begin...

I'm supposed to describe the preparation of 15 liters of .07 M phosphate buffer, pH 7.00 starting from:

a) 1.00 M KH2PO4 and 1.00 M Na2HPO4


What I do know is that the pKa of H2PO4 is 6.82, and I was told this is the only pka necessary, but I don't know why the pka of HPO4 is not necessary.

Online Borek

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Re: Preparing Buffer Solutions
« Reply #1 on: September 17, 2010, 02:57:28 AM »
Solution is dominated by the H2PO4- and its conjugated base, other acids/bases are present in minute quantities.
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Offline Link The Worlds

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Re: Preparing Buffer Solutions
« Reply #2 on: September 17, 2010, 08:15:19 AM »
Okay, that makes sense. But what if instead of 1.00 M KH2PO4 and 1.00 M Na2HPO4, there is 1.2 M K2HPO4 and 2 M H2SO4.


When I plug H2SO4's pka value (-3) into the Henderson-Hasselbech equation, I come out with A/HA = 1 x 1010

So I don't know what to do first.

Offline AWK

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Re: Preparing Buffer Solutions
« Reply #3 on: September 17, 2010, 11:18:14 AM »
Sulfuric acid is much stronger than phosphoric one, so calculate reaction between salt and sulfuric acid first.
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