Question

You will often work with salts of , , and in the laboratory. (All are found in nature, and all are important economically.) If you have a solution containing these three ions, each at a concentration of 0.10 M, what is the order in which their hydroxides begin to precipitate as aqueous NaOH is slowly added to the solution? Formula Enter an integer to indicate order: 1 = first, 2 = second, 3 = last : : :

Answer 1

Answer:

1) Fe³⁺

2) Al³⁺

3) Pb²⁺

Explanation:

Because all of the metal ion concentrations are the same, the order that they precipitate will be dependent on the $K_{sp}$ values of each individual precipitate. The lower the $K_{sp}$ , the less OH⁻ needed in solution to precipitate.

The three $K_{sp}$ values are

$K_{sp}$ = 2.6 x 10⁻³⁹ for Fe(OH)₃,

$K_{sp}$ = 1.9 x 10⁻³³ for Al(OH)₃

$K_{sp}$ = 1.4 x 10⁻¹⁵ for Pb(OH)₂

This means that the ions will precipitate in the following order Fe³⁺ then Al³⁺ then Pb²⁺

Note: if too much OH⁻ is added Al³⁺ will redissolve into solution because of the formation of the soluble complex Al(OH)⁻₄ ($K_{f}$ = 1.1 x 10³³)