Chemistry ksp problem?

Question

BaF2 is slightly soluble in water at 25 deg C. Ksp=4.7E-6. If you have 250 mL of a solution that has [Ba2+]=.2M, how many grams of NaF(s) must you add to make [Ba2+]=.0001M at equilibrium?

Answer 1

BaF2 -->  Ba+2  & 2 F-

K = [Ba+2]  [ F-]^2

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so let's find out what Molarity of  fluoride is needed:

4.7e-6 = [0.001]  [ F-]^2

F2 = 4.7e-3

[F-] = 0.0686 molar

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if it needs 0.0686 moles in one litre, then it needs 1/4 that in 250 ml's=

0.01714 moles of fluoride are needed

0.01714 moles of fluoride can come from 0.0171 moles of NaF

find the grams needed, using the molar mass of NaF:

0.01714 moles of NaF @  41.99 g/mol = 0.718 grams of NaF

your answer (@ 2 sig figs) : 0.72 g NaF are needed

you had 2 sigfig throughout most of these calculations, except for the "0.001" Molar {Ba+2]  , if it wasn't supposed to be 0.0010 Molar, you ought to consider rounding the asnswer off to 0.7 grams of NaF