The solubility of an ionic compound M2X3 (molar mass = 346 g) is 3.9 10-17 g/L. What is Ksp for the compound?

Answer 1

M2 X3 <> 2M3+ + 3X2-

Ksp = (2x)^2 (3x)^3 = 108 x^5

In 1 L we can dissolve 3.9 10-17 g >> 1.13 10-19 moles

Ksp= 108 (1.13 10-19)^5 = 1.96 10 -93

I thing that the solubility is wrong (may be 3.9 10-7) and also the Ksp is wrong. Besides the answer is correct

Answer 2

First convert g/L to mole/L
solubility s =(3.9/346)*10^-17 =1.13*10^-19

.. .. .. .. .. M2X3 <=> 2M(+3) + 3X(-2)
Dissolve .. s
Produce .. .. .. .. .. .. .. 2s .. .. .. .. 3s
At equil .. .. .. .. .. .. . .. 2s .. .. .. .. 3s

Ksp =[M(+3)]^2 *[X(-2)]^3= (2s)^2 *(3s)^3= 4*27*s^5=>
Ksp =108s^5 =108*(1.13*10^-19)^5 =1.99 *10^-93