Potassium hydroxide is used to precipitate each of the cations from their respective solution. determine the minimum concentration of koh required for precipitation to begin in each case.

This is the three cases that help to determine the minimum concentration of KOH required for precipitation 
Part a) 1.5×10^−2 M K CaCl2 
Part b) 2.3×10^−3 M Fe (NO3)2 
Part c) 2.0×10^−3 M MgBr2

a) CaCl2 + 2KOH --> Ca (OH) 2 + 2KCl Ca (OH) 2 <=> Ca^2+ + 2OH^- 
ksp = 1.5*10^-2 + x^2 
4.68*10^-6 = 1.5*10^-2 + x^2 
x= [KOH] = 0.01766 

b) Fe (NO3)2 +2 KOH--> Fe (OH)2 + 2KNO3 
Fe (OH)2 <=> Fe^2+ + 2OH^- 
ksp = 2.3*10^-3 + x^2 
4.87*10^-17 = 2.3*10^-3 + x^2 
x= 1.46*10^-7 

c) MgBr2 + KOH --> Mg (OH) 2 + 2KBr 
Mg (OH) 2 <=> Mg^2+ + 2OH^- 
ksp = 2.0*10^-3 + x^2 
2.06*10^-13 = 2.0*10^-3 + x^2 
x= 1.015*10^-5

"form only at boundaries  between two continental plates." this is the correct answer.