GenChem Solubility Question

[topdent1]

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This is from Kaplan online midterm. I don't seem to understand the explanation much less what the question is asking. Take a look...

The solubility of CaF2 (Ksp = 4 × 10–11) in a 0.1 M solution of Ca(NO3)2 is approximately:

A. 4 × 10–11M

B. 2 × 10–11M

C. 1 × 10–5M

D. 5 × 10–6M

E. None of the above

 

[thebozz1975]

I believe this is the common ion effect. Excuse my bad shorthand:

Originally you have CaF2 --> Ca + 2F
Ksp = [Ca][F]^2 = 4e-11

With the Ca(NO3)2 you have a second source of Ca.

So, the new Ksp = [Ca + x][2x]^2 = 4e-11
you can assume x is very small, so it drops out relative to Ca

so now you have [Ca][2x]^2 = 4e-11
= (0.1)4x^2 = 4e-11
x = 1e-5

this is probably a bit advanced for actual DAT questions.