please derive an expression for the PKsp for the reaction:?

Question

CA3(PO4)2(s) -------------- 3Ca^2(aq)+2PO4^-3(aq)
in terms of the thermodynamic PKap and......

(a) the ionic strength as estimated using the EDHE

(b) the ionic strength as estimated using the Davis expression

Answer 1

For the EDHE you need the mean effective diameter of the ions(α)

As far as I know ionic strength is always I=1/2(ΣCizi^2) and what changes is the expression of the activity coefficient

Kap=γ(+/-)^(2+3)*Ksp =γ(+/-)^5* Ksp =>

pKa=5(-logγ(+/-))+ pKsp

According to EDHE
-logγ(+/-) = 0.512*2*3*sqrt(I) / (1+0.33αsqrt(I))

according to Davies

-logγ(+/-)= 0.512*2*3*( sqrt(I)/(sqrt(I)+1) -0.2I)

substitute and you'll get the answer