Question

if an aqueous solution of BA(NO3)2 is added to 1.0 L of a solution containing 0.05 mol F- and 0.075 mol SO4 2-, which salt will precipitate first?

Answer 1

Look up Ksp. Calculate (Ba^+2) needed to ppt BaSO4 and BaF2. The one requiring the smallest (Ba^+2) will ppt first.

Answer 2

I have the Ksp.
BaSO4 is 1.1 x 10^-10 and BaF2 is 1.8 x 10^-7

but what do you mean calculate (Ba^2+) needed to ppt BaSO4 and BaF2? how do you do that? that's the part i don't understand.

Answer 3

I misspoke. I thought the sulfate and fluoride were given but they are not. So you need to calculate the solubility
BaSO4 ==> Ba^+2 + SO4^-2

Ksp = (Ba^+)(SO4^-2) = 1.1 x 10^-10.
If we let Ba^+2 = S, then SO4^-2 = S and
S^2 = 1.1 x 10^-10. Solve for S = 1.05 x 10^-5 M which is Ba^+ in a saturated solution of BaSO4.

Do the same thing for BaF2 and calculate Ba^+2. Compare.