Question

An acetic acid/ sodium acetate buffer solution is also 0.020 M AlCl3. At what minimum pH will Al(OH)3 (s) precipitate form this solution? What ratio of acetate ion to acetic acid concentrations ([C2H3O2-]/[HC2H3O2]) should be maintained to prevent the precipitation of aluminium hydroxide?

Please show work and address all questions

Answer 1

It should also be known some datas here : Ksp for Al(OH)₃ = 3 x 10^-34

                                                                   Ka for acetic acid = 1,7 x 10^-5

We will go through the reaction involved here:    Al(OH)₃ ===> Al³⁺ + 3OH ^-

                                                                  I : solid    0    0

   C :       solid    x    3x

   E :    solid     x    3x

Since equation of Ksp for this reaction will be;    Ksp = [Al³⁺] [OH^-]³

on putting the values;    3 x 10 ^-34 = (x) (3x)³

3 x 10^-34 = 27 x⁴

solve this for to find out the value of (x) i.e. [Al³⁺] then convert that to [OH^-] , change to [H+] and convert into pH.

Then use the Handerson-Hasselbalch equation -

   pH = pKa + log (base)/(acid)

put the value of pH here you solved, pKa for acetic acid ( pKa = - log Ka ) and thus you will get (base)/(acid) ratio .