Calculate the concentration of ion in the given solution using Ksp

Calculate the concentration of fluoride ions in a saturated solution of chromium fluoride (CrF3). Ksp(CrF3) = 6.6 x 10-11.

Ksp are just a type of equilibrium constant where the Ksp is equal to the the product of the concentration of the dissolved ions raised to the power of their coefficients.  For instance, in this case the equation showing the dissolving of CrF3 would be

                          CrF3 (s)  ---->      Cr3+  (aq)      +     3  F- (aq)

Therefore, the Ksp expression must be

                         Ksp  =    [ Cr3+ ]   [ F- ]3

Given the balanced equation for the dissociation of the CrF3, if the [Cr3+] is given a value of x, then the [F-] must be 3x.  Plugging into the Ksp expression yields

                          Ksp  =    [ Cr3+ ]   [ F- ]3   =    [ x ]   [ 3x  ]3   = 9x4

Since the value of the Ksp is known,  x can be calculated along with [F-] or 3 x.