Question

Will Co(OH)2 precipitate from solution if the pH of a 0.020 M solution of Co(NO3)2 is adjusted to 8.5? ( Ksp of Co(OH)2 is 1.3×10−15 .)

Answer 1

TRUE  CO(OH)₂ will precipitate from the solution if the pH of a 0.020 M solution of CO(NO₃)₂ is adjusted. ; ( TRUE )

Given that:

Ksp of CO(OH)₂ =  1.3 * 10⁻¹⁵

Chemical reaction

CO⁺² + 2OH  ---->  CO(OH)₂

First step : determine the pOH value

pOH of solution = 14 - pH

                          = 14 - 8.5 = 5.5

Also

pOH = - log [ OH⁻ ] = 5.5

Therefore :  [OH⁻] = - Antilog ( 5.5 )

                              = 3.16 * 10⁻⁶

[CO⁺²] = 0.02 given that it is obtained from 0.02 M solution of cobalt nitrate

Next step : determine the Ionic product of cobalt hydroxide

Ionic Product ( IP ) = [CO⁺²] [OH⁻]²

                              = 0.02 * ( 3.16 * 10⁻⁶)²

                              = 1.99 * 10⁻¹¹

Given that the Ionic product is greater than the Ksp of CO(OH)₂ therefore CO(OH)₂ will precipitate

Hence we can conclude that TRUE  CO(OH)₂ will precipitate from the solution if the pH of a 0.020 M solution of CO(NO₃)₂ is adjusted. ; ( TRUE )

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