The question is "Calculate [HY

^{3-}] in a solution prepared by mixing 10.0 mL of 0.01 M VOSO

_{4}, 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 [VO

^{2+}], [VOY

^{2-}], and then [Y

^{4-}] (= [VOY

^{2-}]/([VO

^{2+}] x K

_{6})) and use that to find [HY

^{3-}] which is through the equation ([H

^{+}][Y

^{4-}])/K

_{6}I just don't understand why we are using K

_{6} (derived from pK

_{6} of H6Y

^{2+}) 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