Question

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.

Answer 1

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