Question

the Ksp for magnesium aresenate is 2.1*10^-20at 25 c. what is the molar solubility of Mg3(AsO4)2 at 25c?   a.6.7*10^-3 b. 3.6*10^-4     c. 4.5*10^-5    d.7.0*10^-5    e. 1.4*10^-6

Answer 1

answer : c) 4.5*10^-5

Mg3(AsO4)2 <-------------------> 3 Mg+2   + 2 AsO43-

                                                        3 S             2 S

Ksp = [Mg+2]^3[AsO4-3]^2

2.1 x 10^-20 = (3S)^3(2S)^2

2.1 x 10^-20 = 108 S^5

S = 4.5 x 10^-5

molar solubility = 4.5 x 10^-5