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Topic: Buffer system calculations  (Read 16297 times)

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Offline Anthasci

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Re: Buffer system calculations
« Reply #15 on: June 15, 2012, 08:49:05 AM »
But doesnt the concentration of all forms of the oxalic acid equal the concentration of Zn2+ ions? Which in turn, equals the Ksp divided by the concentration of A2-?

That is assuming Ksp=[Zn2+] [A2-]

Offline Borek

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Re: Buffer system calculations
« Reply #16 on: June 15, 2012, 11:50:24 AM »
Good point, you were right. I misunderstood your understanding ;)
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Offline Anthasci

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Re: Buffer system calculations
« Reply #17 on: June 15, 2012, 12:20:27 PM »
Ok then  ;)

Using the 9.13 equation with Ka1 = 0,0556 and Ka2 = 5,56*10-5 I get a result of 0,0187 g/L

What I did - after calculating the [A2-] from 9.13, I derived the [Zn2+] = s. I then multiplied this by the molar mass of ZnC2O4 and got the result. The solution however, was supposed to be 0,0576 g/L. Are they wrong or did I mix something up?

EDIT: Oh yeah, just noticed about the stepwise thingie ... Got the correct result now. Guess I'll have to work on these derivations a bit  :) I think I understand these system a little better now. I'll always try to write the balance of masses and the balance of charges and go from there (final exam of analitical chemistry on wednesday, it'll be tough, considering we also have the instrumental analysis ...)

One more question though - equation 9.5 on your link - I always count both [H+] and [OH-] ions, since they're both always present in a solution, right? Same would go if it were basic, then I'd just write [H3O+] in the terms of Kw and [OH-]
« Last Edit: June 15, 2012, 12:41:11 PM by Anthasci »

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