Chemistry confusion (saturation, etc.)

[robrobbberts]

I had this problem in my Kaplan study guide:

How does the addition of a small amount of ZnF2 to a saturated solution of PbF2 affect this solution?

A. ZnF2 does not dissolve and PbF2 remains in solution
B. ZnF2 dissolves and PbF2 precipitates.
C. ZnF2 dissolves PbF2 remains in solution
D. ZnF2 does not dissolve PbF2 precipitates

The answer is A, but in my ExamKrackers book it states:

A common ion added to a saturated solution will shift the equilibrium increasing precipitate. A common ion added to a solution that is not saturated will NOT shift the equilibrium, because in an unsaturated solution, there is no equilibrium to shift.

So, why isn't the answer B? wouldn't the ZnF2 add F ions to the solution, thus forcing PbF2 to precipitate?

The passage even states that ZnF2 is more soluble than PbF2.

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[loveoforganic]

I'd agree with B too.

 

[robrobbberts]

So you think the answer I was given was wrong?

 

[loveoforganic]

I'm hesitant to say it's wrong, but I am however sure that nothing I learned in general chem, quantitative analysis, or organic chem agrees with the answer given.

 

[Naijaba]

Does the question give any information about the solvent? PbF2 does not normally dissolve in water.

 

[loveoforganic]

It dissolves to a slight extent. Solubility rules are just approximations.

 

[KaplanZ]

Dear robrobbberts:

To save you any further agony, the correct answer is B. This is a passage in our Mini-Test of the General Chemistry 3 Lesson. Your instinct along with everyone else's is correct.

According to the teacher's version of the Lesson Book, it is most definitely B. If you were not attending a classroom course, does the Lesson-on-Demand tell you the answer is A? If there is an error in course materials, please send me a private message.