chad quiz error?

[joeeeey3]

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using common ion effect, i got 4.2 x 10^-14 but the answer key says its 4.2 x 10^-6
and it doesnt even show solution.

is this an error?

 

[cj7]

4.2 X 10^-6 is correct.

ScF3 --> Sc + 3F-
ksp = [Sc][F-]^3

[Sc] = x
[F-] 3x + .0001, but 3x is negligible, so [F-] = .0001
now plug into ksp equation:

4.2X10^-18 = (x) (.0001)^3
X= 4.2 X 10^-6 M

 

[jaymin8]

ScF3

So that is Ksp= [Sc][F]^3
Given: [F] and Ksp

SO....

4.2*10^-18= [Sc] [1.0*10^-4]^3 -----> multiply exponents
4.2*10^-18= [Sc] [1.0*10^-12] -------> when dividing, you subtract exponents.
Sc=4.2*10^-6

 

[joeeeey3]

oh shnapp haha

thx guys 🙂