Answer:
[OH⁻] = 3,09x10⁻⁷ M
Explanation:
Selective precipitation is a technique of separating ions in an aqueous solution by using a reagent that precipitates one or more of the ions. In this case, you use OH⁻ from potassium hydroxide to precipitate cobalt thus:
Co(OH)₂ ⇄ Co²⁺ + 2 OH⁻ [tex]K_{sp}[/tex] = 5,92x10⁻¹⁵
The [tex]K_{sp}[/tex] definition is:
[tex]K_{sp}[/tex] = [Co²⁺] [OH⁻]²
Knowing [Co²⁺] is 0,0621M:
5,92x10⁻¹⁵ = 0,0621M × [OH⁻]²
Thus, concentration of hydroxide ion required to just initiate precipitation is: [OH⁻] = 3,09x10⁻⁷ M
I hope it helps!
