Question

Groundwater at a temperature of 250C flows through strata containing abundant fluorite (CaF2) mineralization and becomes saturated with respect to that mineral. The equilibrium solubility product, Ksp, for this reaction is 10^-10.4.

a) Calculate the concentrations of Ca2+ and F- once the water has reached saturation with respect to fluorite assuming that prior to entering the mineralized strata the water had an ionic strength of less than 10-5 mol/L and contained negligible Ca2+ and negligible F-. For this part of the problem, neglect activity corrections. (Note that for each mole of CaF2 that is dissolved, you will get one mole of Ca2+ and two moles of F-.

b) Calculate the ionic strength for the groundwater that has the concentrations of Ca2+ and F- that you determined in part a) assuming that Ca2+ and F- are the only major ions in the groundwater. Then calculate the activity coefficients for Ca2+ and F- at this ionic strength. Were you justified in neglecting activity corrections?

c) Repeat the calculations from part a) but this time including activity corrections for water with the ionic strength you determined in part b). Does the calculation including these activity corrections predict more or less dissolution of fluorite?

d) Repeat the calculations from part a) but this time assuming that the groundwater has an initial ionic strength of 0.01 and an initial concentration of Ca2+ of 0.55 x 10^-3 mol/L. (Note that you will need to use the extended Debye-Huckel equation for activities at this ionic strength. Also, remember that each mole of fluorite that dissolves will add one mole of Ca2+ and two moles of F- to what is already in solution.)