Why is K+ larger than Ca^2+?

Who knows the [ca2+] of CaF2 in pH 3.18 water ?

• caf2---->ca2+ + 2F- at aout pH 3.18 water, F- exist HF and F- in the ratio 1:1 coz pH=pka Ksp= [ca2+][F-]^2 [Ca2+]=x Can the equation be Ksp=(x)(x/2)^2 right?? i can't understand why [F-] is x/2 in that part.

• Answer:

  F- takes part in two different reactions. So the concentration at equiliibrium should be satisfying both Ksp and Ka and thus the total amount of CaF2 that will dissolve will be a bit more than in simple water (part of the F is trapped as HF) That's why if [Ca+2]=x then [F-] is not 2x, but [F-]total=2x and [F-]total=[F-]+[HF] but you said that pH=pKa, thus [F-]=[HF] thus [F-]total=[F-]+[HF]= [F-]+ [F-]= 2[F-] and we said [F-]total=2x=> Thus 2[F-]=2x => [F-]=x thus Ksp=x*x^2=x^3