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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: BatMiteDimension on November 18, 2012, 07:56:41 PM

Title: Calculating [HY3-] in a solution prepared with EDTA, why do we use pK6?
Post by: BatMiteDimension on November 18, 2012, 07:56:41 PM

The question is "Calculate [HY^3-] 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⁺], [VOY^2-], and then [Y^4-] (= [VOY^2-]/([VO²⁺] x K₆)) and use that to find [HY^3-] which is through the equation ([H⁺][Y^4-])/K₆

I just don't understand why we are using K₆ (derived from pK₆ of H6Y²⁺) and using that equation to find HY^3-. Is this what we do if we want to find any of the fractional compositions of EDTA or just for HY^3-? What made us use Y^4- 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 :)

Title: Re: Calculating [HY3-] in a solution prepared with EDTA, why do we use pK6?
Post by: Borek on November 19, 2012, 04:43:41 AM

Quote from: BatMiteDimension on November 18, 2012, 07:56:41 PM

[Y^4-] (= [VOY^2-]/([VO²⁺] x K₆))

K₆ or K_f?

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 K_a6 and K_f?

Title: Re: Calculating [HY3-] in a solution prepared with EDTA, why do we use pK6?
Post by: BatMiteDimension on November 19, 2012, 11:10:03 AM

Oh, I'm sorry! You're correct, it is K_f in [Y^4-] (= [VOY^2-]/([VO²⁺] x K_f))

K_f for VOY^2-= 10^18.7
pK₆ for H₆Y²⁺=10.37

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