1. The problem statement, all variables and given/known data

How many moles of solid sodium fluoride should be added to 1.0 L of a saturated solution of barium fluoride, BaF_{2}, at 25^{O}C to raise the fluoride concentration to 3.0 x 10^{-2} mol/L? The solubility product constant for BaF_{2} is K_{sp} = 1.7 x 10^{-6} at 25^{O}C.

3. The attempt at a solution

BaF_{2} ---> Ba^{2+} + 2F^{-}

Initial Concentrations: 0, 0
Change: + x, + 2x

Therefore:

1.7 x 10^{-6} = x(2x)^{2}

x = 0.00752 M.

So: [F^{-}] = 0.01504 mol/2 L.

I thought to have the desired fluoride concentration, only 1.5 x 10^{-2} mol would need to be added. However, the correct answer is