What is the minimum value for Ksp for calcium fluoride?

The corrosion of stainless steel containers in the proposed Yucca Mountain nuclear waste repository in Nevada is a potential problem. One potentially corrosive ion is fluoride, which appears to be removed from solution by precipitation as calcium fluoride. In test measurements, the concentration of calcium ion was found to be 0.00110 M, and the concentration of fluoride ion was found to be 0.0218 g/L. What is the minimum value for Ksp for calcium fluoride based on these measurements ? Use units of moles and liters in your calculation.

What is the minimum value for Ksp for calcium fluoride?
[Ca2+] = .00110M


[F-1] = .0218 g/L





Ca2+ + 2F-1 -%26gt; CaF2





.0218/18.9984 = .001147M





Ksp = [.00110][.001147]^2 = 1.447e-9
Reply:Well is they found F-





at 0.00110M





and 0.0218g/L * (mole / 19 g) = 0.0114 M





then the minimum Ksp has to be between





CaF2 --%26gt; Ca+2 + 2F-





Let x = F-





1/2x = Ca+2





If F- were in the range of (0.0110) and (0.0114)





Ksp CaF2 = ((0.0110)/2)(0.0110)^2) = 6.7 * 10^-7


and





Ksp CaF2 = ((0.0114/2)(0.0114)^2) = 7.4 * 10^-7





the lowest of these is 6.7 * 10^-7 so that has to be the answer.