Jul 23, 2009
33
0
Status
Pre-Dental
So.. this is a problem from my Gchem txtbook.. and i do have the solution for it on the back of the book, but not HOW to solve it. i'm really bad with setting up solubility equilibrium stuff so please help me!


Q: CaF2 is added to a 0.1M Ca(NO3)2 solution. At what concentration of F- will CaF2 begin to precipitate? (Note: the Ksp of CaF2 is 4e-11)




the answer is "2e-5" and I just don't know how =( someone help me out? thanks!
 
Jun 10, 2009
83
0
Status
OK

First you go off with
Q: CaF2 is added to a 0.1M Ca(NO3)2 solution. At what concentration of F- will CaF2 begin to precipitate? (Note: the Ksp of CaF2 is 4e-11)

you always take the ksp for the stuff the askin for so write ur eq out


CaF2 --> Ca2+ + 2F-

Ksp=[Ca2+] [F-]^2

plug it in

4e-11= [.1][F-]^2

4e-11/1e-1=[F-]^2

(4e-10)^1/2 = F^2
2e-5=F-
 
OP
R
Jul 23, 2009
33
0
Status
Pre-Dental
thank you so much!! i thought i had to used another calculation with the 0.1M of Ca(NO3)2 to find out the concentration of Ca2+ in here, but realized that the mole ratio is the same so i can just use the same molarity given in the equation. THANKS AGAIN! :)