Question

A solution containing sodium fluoride is mixed with one containing calcium nitrate to form a solution that is 0.015M in NaF and 0.010M I. Ca(NO3)2. Calculate the reaction quotient (Q) for the possible cross product CaF2, and compare Q to the solubility product constant of CaF2 to determine if a percipitate will form.

Answer 1

According to the reaction equation:

CaF2(s) ⇄  Ca2+(aq)  +  2F-(aq)

So the Q expression will be = [Ca2+][F-]^2 

and we will neglect the [ CaF2] as it is solid 

so, by substitution:

when we have [Ca2+] = 0.01

and [F-] = 0.015

Q = 0.01 * (0.015)^2

   = 6.3 x 10^-2

by comparing Q value which is 6.3 x 10^-2 and Ksp value which is 1.8 x 10^-5

So, we can see that Q > Ksp 

-that means precipitation will form as ion concentration are greater than

equilibrium concentration So, the reaction will proceed in reverse to achieve 

equilibrium . and the solution will be then supersaturated.