Predict Ksp for each compound?

Question

CaF2 ; 1.70mg/ 100 mL


SrC2O4 ; 1.58 * 10^-7 mol/L

Can't figure out how to do these! Help please!

Answer 1

Actually we have to calculate based on the given solubilities and not to predict.

First of all convert it to molar solubility. For the second one it it already in that format, so we need to change only the one of CaF2.

The molecular weight of CaF2 is:40+2*19 = 78
So you have 1.70 mg/ 100 mL = 1.7 *10^-3 g/100mL =
= 1.7*10^-2 g/L =(1.7*10^-2)/78 mole/L =2.18*10^-4 M

Let's call this molar solubility s. Then
.. .. .. .. .. .. .. .. CaF2 <=> Ca+2 +2F-
Dissolve .. .. .. .. s
Produce .. .. .. .. .. .. .. .. .. .. s .. .. .. 2s
At Equil .. .. .. .. .. .. .. .. .. .. . s .. .. . 2s

Ksp = [Ca+2][F-]^2 = s(2s)^2 =4s^3 =4*(2.18*10^-4)^3 =4.14*10^-11

.. .. .. .. .. .. .. ..Sr C2O4 <=> Sr+2 +C2O4(-2)
Dissolve .. .. .. .. s
Produce .. .. .. .. .. .. .. .. .. .. s .. .. .. .. .. s
At Equil .. .. .. .. .. .. .. .. .. .. . s .. .. .. .. .. s

Ksp = [Sr+2][C2O4(-2)] = s^2= (1.58*10^-7)^2 =2.50*10^-14