Question

explain why the relative solubility of a salt is not necessarily the same as the relative Ksp value.

Answer 1

Ksp value depends on solubilities S

The relation between Ksp and S changes based on valence or charge of ions formed

example salts like NaCl , KCl   have Ksp = S^2

salts like BaCl2 which gives 3 ions ( 1Ba2+ and 2Cl-)   Ksp = [Ba2+] [Cl-] = ( S) ( 2S)^2 = 4S^3

Thus if solubility of salt A is higher than other salt B that doesnot indicate Ksp value of A is higher than Ksp value of B

exmple CdCO3 has   Ksp 5.2 x 10^ -12

Cd(OH)2 has Ksp 2.5 x 10^ -14

CdCO3 Ksp > Cd(OH)2 Ksp

now we check solubilities

Ksp of CdCO3 = S^2 = 5.2 x 10^ -12

S = 2.28 x 10^ -6 M

Ksp fo Cd(OH)2 = 4S^3 = 2.5 x 10^ -14

S = 1.84 x 10^ -5 M

Hence Ksp of CdCO3 > Ksp fo Cd(OH)2

but solubilities of Cd(OH)2 is higher than CdCO3         ( S for Cd(OH)2 > S for CdCO3)

Thus relative solubilities is not necessarily same as relative Ksp