October 19, 2019, 03:58:20 PM
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Topic: Calculating [HY3-] in a solution prepared with EDTA, why do we use pK6?  (Read 3558 times)

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Offline BatMiteDimension

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The question is "Calculate [HY3-] in a solution prepared by mixing 10.0 mL of 0.01 M VOSO4, 9.90 mL of 0.01 EDTA, and 10.0 mL of buffer with a pH of 4.00".

The answer is to first find [VO2+], [VOY2-], and then [Y4-] (= [VOY2-]/([VO2+] x K6)) and use that to find [HY3-] which is through the equation ([H+][Y4-])/K6

I just don't understand why we are using K6 (derived from pK6 of H6Y2+) and using that equation to find HY3-. Is this what we do if we want to find any of the fractional compositions of EDTA or just for HY3-? What made us use Y4- instead of anything else?

Thank you!

P.S.: I posted this question yesterday but I did not format the post properly. I reposted it here now with proper formatting :)

Offline Borek

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Re: Calculating [HY3-] in a solution prepared with EDTA, why do we use pK6?
« Reply #1 on: November 19, 2012, 04:43:41 AM »
[Y4-] (= [VOY2-]/([VO2+] x K6))

K6 or Kf?

Somehow I am not convinced method you outlined will give a correct result (but I can be easily wrong, something is not clicking). Can you list Ka6 and Kf?
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Offline BatMiteDimension

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Re: Calculating [HY3-] in a solution prepared with EDTA, why do we use pK6?
« Reply #2 on: November 19, 2012, 11:10:03 AM »
Oh, I'm sorry! You're correct, it is Kf in [Y4-] (= [VOY2-]/([VO2+] x Kf))

Kf for VOY2-= 1018.7
pK6 for H6Y2+=10.37

I still don't understand why we use K6 for the rest of the question though.

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