[elaineiiv]

 well, this is an Ap problem I've been trying to solve....sadly it's been a while since I last had to deal with equilibrium concentrations and am a bit fuzzy on the subject. The problem goes as follows:

   MgF₂^->_<-Mg⁺²+2F^-
In a saturated solution of MgF₂ at 18^oC, the concentration of Mg⁺² is 1.21x10^-3 molar. The equilibrium is represented by the equation above.

Calculate the equilibrium concentration of Mg⁺² in 1.000L of saturated MgF₂ solution at 18^oC to which 0.100 mole of solid KF has been added. The KF dissolves completely. Assume the volume change is negligible.


My problem is that I'm not exactly sure where to start solving the problem from. Also, I don't know if I can use the given concentration of Mg⁺² for the whole substance MgF₂ as that's where all the magnesium's comming from. Pretty much I've worked myself into a corner and I'd love it if someone could give me an idea on what to do first.
I did try and solve the problem by using a table with 1.21x10^-3 and 2x(1.21x10^-3) as my initial concentrations for Mg⁺² and F^- respectively. The change in value was x=.1 and my final answer .101M. somehow I'm not sure I'm right.

On the note of Ap exams, does anybody know where I can ges answers for free responses from previous years, so that I can check my work?
 

[Borek]

Concentration of Mg²⁺ allows you to calculate Kso for MgF₂. Now you add excess of fluoride - so just use Kso to calculate new concentration of Mg²⁺. That's all.