GenChem Chad Quiz Question

[Ame535]

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Hi,
Can someone please explain to me how to solve this question:

What is the maximum concentration of fluoride ions that could be present in 0.032M Ba(NO3)2 (ksp,BaF2 = 3.2*10^-8)?

The solution states:
BaF2(s) --> Ba2+(aq) + 2F-(aq)
Ksp = [Ba2+] [F-]^2
3.2*10^8 = (0.032) [F-]^2
....... then the answer is [F^-]= 1.0*10^-3M


My question is how come the concentration for [F-]^2 was not 2x^2?

 

[FeralisExtremum]

[F-] ions are not dissolving in the solution from BaF2. Rather, we are trying to figure out how how much F could combine with the existing concentration of Ba in solution before exceeding the Ksp and precipitating. So we wouldn't use [2X] in place of [F-] because the F- ions aren't coming from BaF2 dissolving. We use the Ksp expression Ksp = [Ba2+] [F-]^2 and solve for [F-] directly.

By the way, the Ksp of BaF2 should be 3.2*10^-8

 

[Ame535]

[F-] ions are not dissolving in the solution from BaF2. Rather, we are trying to figure out how how much F could combine with the existing concentration of Ba in solution before exceeding the Ksp and precipitating. So we wouldn't use [2X] in place of [F-] because the F- ions aren't coming from BaF2 dissolving. We use the Ksp expression Ksp = [Ba2+] [F-]^2 and solve for [F-] directly.

By the way, the Ksp of BaF2 should be 3.2*10^-8

Oh ok, but when solving for molar solubility we will have [F-]^2 as 2x^2 because it's dissolving?

Yeah you are right I will edit it.