pH range buffer

Question

A solution contains  M Pb2+ and  M Al3+. Calculate the pH range that would allow Al(OH)₃ to precipitate but not Pb(OH)₂.

The Ksp values for Al(OH)₃ and Pb(OH)2 are 4.6x10^-33 and 1.43x10^-20, respectively.

Answer 1

Hello Eli!

For this question, let's write the equations of solubility:

Al(OH)_3(s) ↔ Al³⁺_(aq) + 3OH^-_(aq)

Q = [Al³⁺] * [OH^-]³

Pb(OH)_{2 (s)} ↔ Pb²⁺_(aq) + 2OH^-_(aq)

Q = [Pb²⁺] * [OH^-]²

If Q > Ksp then it will precipitate.

For the Al(OH)₃ we'll have:

[Al³⁺] * [OH^-]³ > 4.6 . 10^-33

0,45 * [OH^-]³ > 4.6 . 10^-33

[OH^-]³ > 1.022 . 10^-32

[OH^-] > 2.17 . 10^-11

pOH > 10.66

pH > 3.33

For the Pb(OH)₂ we'll have:

[Pb²⁺] * [OH^-]² > 1.43 . 10^-20

0,26 * [OH^-]² > 1.43 . 10^-20

[OH^-]² > 5.5 . 10^-20

[OH^-] > 2.34 . 10^-10

pOH > 9.63

pH > 4.37

For the

3.33 < pH < 4.37 only the Al(OH)₃ will precipitate