Chemistry help!!!?

Question

If you have a solution containing the three ions Fe3+, Pb2+, and Al3+ , each at a concentration of 0.10 M, what is the order in which their hydroxides precipitate as aqueous NaOH is slowly added to the solution?

Answer 1

You ought to provide the Ksp values for the hydroxides.
Precipitation will occur when the respective concentration product exceeds the Ksp. So the minimum [OH-] for each one to precipitate is


For Fe(OH)3 <=> Fe+3 +3OH-

Ksp=[Fe+3][OH-]^3 => [OH-]= (Ksp/[Fe+3])^(1/3) =.
[OH-] =((2.79*10^-39)/0.1)^(1/3) = 3.03*10^-13 M

For Pb(OH)2 <=> Pb+2 + 2OH-

Ksp =[Pb+2][OH-]^2 => [OH-]=squareroot ( [Ksp/[Pb+2]) =>
[OH-] = SQRT((1.43*10^-20)/0.1)= 3.78 *10^-10 M


For Al(OH)3 <=> Al+3 +3OH-

Ksp=[Al+3][OH-]^3 => [OH-]= (Ksp/[Al+3])^(1/3) =.
[OH-]= ((3*10^-34)/0.1)^(1/3) =1.44*10^-11 M

So first is Fe(OH)3 then Al(OH)3 and finally Pb(OH)2