Question

1) The concentration of the aluminum ion in a test solution is 0.10M. Using the Ksp for aluminum hydroxide (3. x 10^-34), calculate the concentration of hydroxide needed to precipitate aluminum hydroxide. From the concentration of the hydroxide, calculate the pH of the solution needed to cause precipitation of aluminum hydroxide.

2) If excess hydroxide is added to the solution in question 1, a complex will form that will dissolve the solid aluminum hydroxide, Al(OH)3. Write the chemical equation that would be associated with the complex that will form from adding additional sodium hydroxide solution to the solid Al(OH)3.

Answer 1

Al(OH)3 --> Al^+3 + 3OH^-

Ksp + (Al^+3)(OH^)^3.
You know Ksp and (Al^+3) from the problem. Calculate (OH^-).
This will be the OH needed to just cause pptn of the solid. Convert this OH to pOH, then to pH.

2. Al(OH)3 + OH^- ==> Al(OH)₃^-

Answer 2

I made a typo in the last equation. It should be
Al(OH)3 + OH^- ==> Al(OH)₄^-

Answer 3

pH = 3.16

Al(OH)3 + OH^- ==> Al(OH)4-