[Mal1ce]

Co(OH)₂ with 0.1 M of NH₃

Co(OH)₂ Co²⁺ + 2OH^-
Co²⁺ + 4NH₃  Co(NH₃)₄²⁺
NH₃ + H₂O  NH₄⁺ + OH^-
2H₂O  H₃O⁺ + OH^-

[OH^-] = 2([Co²⁺] + [Co(NH₃)₄²⁺] + [NH₄⁺] + [H₃O⁺])
0.1 = [NH₃] + 4[Co(NH₃)₄²⁺] + [NH₄⁺]

I was wondering if someone can check my work.  I'm not sure if I wrote the correct mass balance equation.

[Borek]

Charge balance is wrong (why do you multiply concentrations of NH₄⁺ and OH^- by 2?), ammonia mass balance is OK.

[Mal1ce]

Are you saying that the charge balance is the same as the mass balance equation for [OH^-]?

[OH^-] = 2[Co²⁺] + 2[Co(NH₃)₄²⁺] + [NH₄⁺] + [H₃O⁺])

[Pradeep]

your first equation is not a mass balance equation. Even though it is a charge balance equation that equation  also not correct. Mass balance is a equation which relates the total mass and speciation. second equation is correct.

[Borek]

Are you saying that the charge balance is the same as the mass balance equation for [OH^-]?

[OH^-] = 2[Co²⁺] + 2[Co(NH₃)₄²⁺] + [NH₄⁺] + [H₃O⁺])

Looks like.