Help please: Chemistry....im horrible.

Question

Consider a salt of composition, A2B3. For this salt, pKsp = 19.94. What is the logarithm of the molar concentration of this salt?

i've seen it done but im not understanding how to get rid of x5

Answer 1

A2B3 <------> 2 A3+ + 3 B2-

Let x = mol/L of A2B3 that dissolve : this will give us 2 x mol/L of A3+ and 3 x mol/L of B2-

Ksp = 10^-19.94 = 1.15 x 10^-20 = ( 2x)^2( 3x)^3 = 4x^2 ( 27x^3) =

=108 x^5

x = 4.03 x 10^-5 M ( molar solubility)

log 4.03 x 10^-5 = - 4.39

- log 4.03 x 10^-5 = 4.39

Answer 2

for a salt A2B3 ------- 2 A + 3 B   in terms of an unknown concentration X then the Ksp = (2X)^2( 3X ) ^3  = 108 X^5the Ksp = 1.15 X 10^-20

so 108X^5 = 1.15 X 10^-20

X^5  = 1.06 X 10^-22         so X = 4.02 X 10^-5 M in A2B3

-log solubility =  p 5-log 4.02 = 4.4 psol=4.4