[tpowell35]

calculate the concentration of uncomplexed M2+ in solution that is 15mM in M2+, 100mM in EDTA and buffered to a pH of 6.0. Given: Log Kf for the MY2- complex is 18.0 and the pKa values for the stepwise ionization of EDTA are: 0.0, 1.5, 2.0, 2.69, 6.13 and 10.37 respectively.

So i found alpha y4- to equal: 1.8 x 10^-5

since the pH is fixed, i used conditional formation constant (Kf' = (αy4-)(Kf) = [MY2-]/[M2+][EDTA], with excess EDTA = 0.085 M.

so: (10^18.0)(1.8 x 10^-5) = [0.015]/

• [0.085]

x= 9.8 x 10^-15

can anyone confirm this accuracy? or, if it is incorrect, tell me where i went wrong?

thanks, any help is greatly appreciated- i've got a beast of a QA test tomorrow!

[Borek]

Are you sure about these dissociation constants? They don't make much sense to me. As far as I can tell 10.3 is pK_a4, not pK_a6.

[tpowell35]

those are the values given on the sample exam and in the 8th edition of quantitative chemical analysis by harris.

[Borek]

At least it checks like a correct order of magnitude to me.