Question

Freshly prepared aluminium and magnesium hydroxides are stirred vigorously in a buffer solution containing `0.25M` of ammonium chloride and `0.05M` of ammonium hydroxide. Calculate the concentration of aluminium and magesium ions in solution `(K_(b) NH_(4)OH = 1.8 xx 10^(-5), K_(sp) Mg(OH)_(2) = 6 xx 10^(-10), K_(sp) A1(OH)_(3) = 6 xx 10^(-32)`.

Answer 1

`pOH = pK_(b) + log ("salt"//"base")`
`= 4.7447 + log (0.25//0.05) = 5.4431`
`[overset(Θ)OH] = 3.6 xx 10^(-6)M`
`K_(sp)of A1(OH)_(3) = 6 xx 10^(-32) = [A1^(+3)] [overset(Θ)OH]^(3)`
`[A1^(3+)] = (6xx10^(-32))/((3.6xx10^(-6))^(3)) = 1.28 xx 10^(-15)M`
`K_(sp) = Mg [OH]_(2) = 6 xx 10^(-10) = [Mg^(2+)] [overset(Θ)OH]^(2)`
`[Mg^(2+)] = (6xx10^(-10))/((3.6 xx 10^(-6))^(2)) = 46.296M`