gen chem

[BrownieDDD]

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CaF2 is added to a 0.1M Ca(NO3)2 solution. At what concentration of F- will CaF2 begin to precipitate? (Note: the Ksp of CaF2 is 4e-11)




the answer is "2e-5" and I just don't know how....

kaplans's way

f = sqrt (4 * 10^ 11/ 0.1)



isn't the ksp suppose to be
ca (f)^2= ksp
x (2x)^2= 4 * 10^-11
w/ common ion effect, the 2x should really be 2x + 0.1, and thus, the 2x is neglible bc common ion effect....so it's just 0.1

x (0.1)^2= 4* 10^-11

then solve for x for the answer...

but why didnt' kaplan do this?

 

[premac]

CaF2 is added to a 0.1M Ca(NO3)2 solution. At what concentration of F- will CaF2 begin to precipitate? (Note: the Ksp of CaF2 is 4e-11)




the answer is "2e-5" and I just don't know how....

kaplans's way

f = sqrt (4 * 10^ 11/ 0.1)



isn't the ksp suppose to be
ca (f)^2= ksp
x (2x)^2= 4 * 10^-11
w/ common ion effect, the 2x should really be 2x + 0.1, and thus, the 2x is neglible bc common ion effect....so it's just 0.1

x (0.1)^2= 4* 10^-11

then solve for x for the answer...

but why didnt' kaplan do this?

it's actually
ca (f)^2 = ksp
(2x + 0.1) (x^2) = 4e-11
and 2x is negligible.. so...
0.1(x^2) = 4e-11
x = sqrt (4e-11 / 0.1)

what they gave you was 0.1 M of Ca(NO3)2 solution.
that would mean 0.1 M of Ca for common ion effect, not F.

hope that helps.