You need the Ksp or Kb whichever you have listed in your book… look it up. Probably Ksp because Ni(OH)2 is a solid.

  Ni(OH)2(s)  <--> Ni(2+)  +  2 OH(-)

  Ksp  =  [Ni(2+)] [OH(-)]^2

 so ...   [Ni(2+)]  =  Ksp / [OH(-)]^2

the solubility of Ni(OH)2 = [Ni(2+)]

1. pH = 8.0 means pOH = 14- ph = 6 === [OH-] = 10^-6

   so … from above … [Ni(2+)] = Ksp / [OH(-)]^2

                 [Ni(2+)]  =  Ksp / (10^-6)^2

   …. after you get Ksp just use a calculator.

2 and 3 are done the same way.

You need the Ksp or Kb whichever you have listed in your book… look it up. Probably Ksp because Ni(OH)2 is a solid.

  Ni(OH)2(s)  <--> Ni(2+)  +  2 OH(-)

  Ksp  =  [Ni(2+)] [OH(-)]^2

 so ...   [Ni(2+)]  =  Ksp / [OH(-)]^2

the solubility of Ni(OH)2 = [Ni(2+)]

1. pH = 8.0 means pOH = 14- ph = 6 === [OH-] = 10^-6

   so … from above … [Ni(2+)] = Ksp / [OH(-)]^2

                 [Ni(2+)]  =  Ksp / (10^-6)^2

   …. after you get Ksp just use a calculator.

2 and 3 are done the same way.