Question

Solid sodium hydroxide is slowly added to 150 mL of a 0.0466 M cobalt(II) acetate solution. The concentration of hydroxide ion required to just initiate precipitation is M.

Answer 1

Co(CH₃COO)₂ (aq) + 2 NaOH (aq) --> Co(OH)₂ (s) + 2 Na(CH₃COO)

It is a double Replacement reaction.

Ksp of cobalt(II) hydroxide, Co(OH)₂ is 1.6 x 10^-15

Source: http://bilbo.chm.uri.edu/CHM112/tables/KspTable.htm

Assuming total volume = 150 mL after addition of solid NaOH,

Co(OH)₂ (s) <à Co²⁺ + 2 OH-

[Co²⁺] just before precipitation = Concentration of Co (II) acetate solution = 0.0466 M

Ksp = [Co²⁺][OH-]₂

1.6 x 10^-15 = 0.0466 * [OH-]²

[OH-]² = 3.43 x 10^-14

[OH-] = 1.85 x 10^-7 M

[OH-] required to just initiate precipitation is 1.85 x 10^-7 M