CaF2-----> Ca+2 + 2F-, make the concentration of ca+2=x, this means that the [2F-]=2x. plugging into the ksp eqation: (x)*(2x)^2=ksp=4*10^-11, solve for x, you get x=10^-11/3 which is the concentraion of the ca+2.

For the common ion affect, Ca(NO3)2-----> ca+2 + 2NO3. so the ksp, or solubility, is equal to x(from above)*(.1, the concentration of the NO3 soln)^2. so, (10^-11/3)*(10^-2)^2=10^-15/3=10^-5.

hope this helps

JohnDoeDDS said:

Can you guys explain this please. I'm having trouble following the Kaplan explanation and I understand ther eis a common ion effect ehre. Thanks

The Solubility of CaF2 (Ksp=4X10^-11) in a .1M solution of Ca(NO3)2 is approximately:

A) 4X10^-11 M
B) 2X10^-11 M
C) 1X10^-5 M
D) 5X10^-6 M

I can't answer your first question. But with the second, you will need to know the principle of common ion effect. If you add ion X to a solution already with that ion, then the concentration of that ion is not felt nearly as much as it would if there was no solution to begin with. That might sound confusing, hopefully someone else can explain it better, but essentially the concentration of what you refer to as x is negligable.

#2
[Ag+]²[SO4 2-] simply means (concentration of Ag+)^2(Conc. of SO4 2-)

when you know conc. of Ag+ is 0.1 M you don't need to multiply it by anything (i.e. (0.1)^2). If you start with 0 Ag+ and X M Ag2SO4 because you know for every mole of dissociated Ag2SO4 you get 2 moles of Ag+ you need to multiply by 2 (i.e. (2 * dissociated amnt S)^2))....

#2
[Ag+]²[SO4 2-] simply means (concentration of Ag+)^2(Conc. of SO4 2-)

when you know conc. of Ag+ is 0.1 M you don't need to multiply it by anything (i.e. (0.1)^2). If you start with 0 Ag+ and X M Ag2SO4 because you know for every mole of dissociated Ag2SO4 you get 2 moles of Ag+ you need to multiply by 2 (i.e. (2 * dissociated amnt S)^2))....

You guys the f*cking best! I love you! I used to crack these problems easily in my gen chem class and thats exactly how I did it. The kaplan explanation just confused me.

But here is a follow up question:
If you know the M of Ag2SO4. I will get Ksp= (2x)^2 * X... And then what? I dont know the KSP I just know the concentration of the Ag2SO4 where will I go from there? Thanks again eveyone you are great!