In: Chemistry
Calculate the solubility of LaF3 (Ksp = 2 x 10-19) in grams
per liter in: a) pure water b) 0.010 M KF solution
Solubility in pure water : Let S (mol/L) be the solubility
LaF3 La3+ + 3F-
1 mole of LaF3 produces 1 mole of La3+ and 3 moles of F -
concentration of [ La3+ ] = S and
concentration of [F-] = 3S
Solubility product constant ,Ksp = [ La3+ ] [F-]3
2x10-19 = S x (3S)3
27S4 = 2x10-19
S = 9.28x10-6 mol/L
Therefore the solubility in pure water is 9.28x10-6 mol/L
Solubility in 0.010M KF : Let S (mol/L) be the solubility
KF K+ + F-
0.010M 0.01 M 0.01 M
So [F-] = 3S + 0.01
[ La3+ ] = S
Solubility product constant ,Ksp = [ La3+ ] [F-]3
2x10-19 = S x (3S + 0.01)3
2x10-19 = S x ( 0.01)3 Since solubility is very low, 3S+0.01 can be taken as 0.01
S = 2.0x10-13 mol/L
Therefore the solubility is 2.0x10-13 mol/L