A saturated solution of BiI3 has an I- concentration of 3.96x10^-5 M. What is the value of Ksp for BiI3?

Question

Answer choices are:

1) 2.7 x 10^-19

2) 6.6 x 10^-17

3) 4.2 x 10^-25

4) 8.2 x 10^-19

5) 9.1 x 10^-20

6) 3.0 x 10^-20

7) 5.9 x 10^-18

I already know the answer (#4), but I have no clue as to how to arrive at it. Normally for this problem I would follow the procedure Ksp = (3x)^3(x)

What am I doing wrong?

Thanks!

Answer 1

Bi3 ---------> Bi3+     +      3I-

x                       x                  3x

conc I-  = 3.96x10-5M =3x

conc Bi3+ = 3.96x10-5/3 = 1.32x10-5 =x

kps=x*(3x)^3 = 1.32x10-5 *  (3.96x10-5)^3 = 8.2x10-19