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.
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
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
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