Question

I don't really understand how to do this problem, or where to start exactly. Can someone explain to me where to start and how I can solve the problem in a step by step process?
Thank you in advance!


1. An acetic acid / sodium acetate buffer solution is also 0.020 M AlCl3. The Ksp of aluminum hydroxide is 3 x 10^-34.

a) At what minimum pH will Al(OH)3 (s) precipitate form this solution?

b) What ratio of acetate ion to acetic acid concentrations ([C2H3O2^-]/[HC2H3O2]) should be maintained to prevent the precipitation of aluminum hydroxide? (Ka acetic acid = 1.7x10^-5).

Answer 1

...........Al(OH)3 ==> Al^3+ + 3OH^-
I.........solid.........0.......0
C.........solid.........x.......3x
E.........solid.........x.......3x

Substitute the E line into the Ksp expression for Al(OH)3 and solve for x = (Al^3+). Convert that to OH^-, change to H^+ and convert to pH.

Then use the Henderson-Hasselbalch equation. pH = pKa + log (base)/(acid)
The pH from part 1 goes in for pH here, pKa is that for acetic acid and solve fr (base)/(acid) ratio.