Question

The molar solubility of Iron(III) hydroxide, Fe(OH)₃ is 1.01 x 10^-10 M. What is the solubility product constant, K_sp for this ionic compound in water?

My teacher got 2.81*10^-39. How is that correct?

Answer 1

Solution :-

Given data

Solubiliy of the Fe(OH)3 = 1.01*10^-10 M

Dissociation equation of the Fe(OH)3 is as follows

Fe(OH)3 -----> Fe³⁺ + 3OH^-

Since mole ratio of the Fe(OH)3 to Fe³⁺ is 1 :1 therefore concentration of the Fe³⁺ =1.01*10^-10 M

And mole ratio of the Fe(OH)3 to OH- is 1 :3 therefore concentration of the OH- = 3* [Fe(OH)3]

[OH-] = 3*1.01*10^-10 M

           = 3.03*10^-10 M

Lets write the ksp equation for this equation

ksp = [Fe³⁺][ OH^-]³

now put the concentration values in the ksp formula

ksp = [Fe³⁺][ OH^-]³

ksp = [1.01*10^-10][ 3.03*10^-10]³

ksp = 2.81*10^-39

Therefore the answer given by the teacher is correct