G.Chem Question!!!!

[researchlove]

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!

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[panda22]

would you make an ICE table??

 

[coffinhaed]

watch 5-2 of the chad videos he goes over it the second problem he does in it. hope that helps

http://www.coursesaver.com/showthread.php?t=39

 

[coffinhaed]

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-

 

[researchlove]

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!