Question

1- Solid cobalt(II) nitrate is slowly added to 150 mL of a 0.0337 M potassium sulfide solution. The concentration of cobalt(II) ion required to just initiate precipitation is  M.

2- Solid nickel(II) nitrate is slowly added to 175 mL of a 0.0657 M sodium cyanide solution. The concentration of nickel ion required to just initiate precipitation is  M.

3-Solid sodium hydroxide is slowly added to 150 mL of a 0.0496 M lead acetate solution. The concentration of hydroxide ion required to just initiate precipitation is  M.

Answer 1

1)

The dissociation equillibrium

CoS(s) <--------> Co²⁺(aq) + S^2-(aq)

Ksp = [Co²⁺][S^2-] = 3.0×10^-26 M²

given concentration of S^2- = 0.0337M

[Co²⁺] × 0.0337M = 3.0×10^-26 M²

[Co²⁺] = 3.0×10^-26 M²/0.0337M = 8.90× 10^-25M

Therefore,

The concentration of Cobalt(II) ion reaquired to just initiate precipitation = 8.90×10^-25M

2) solubility equillibrium of Ni(CN)2

Ni(CN)2(s) <-------> Ni²⁺(aq) + 2CN^-(aq)

Ksp = [Ni²⁺] [CN^-]² = 3.0×10^-23 M³

given concentration of CN^- = 0.0657M

[Ni²⁺] × (0.0657M)² = 3.0×10^-23M³

[Ni²⁺] = 6.95×10^-21M

Therefore,

The concentration of Nickel ion required to just initiate precipitation = 6.95×10^-21M

3) Solubility equillibrium of Pb(OH)2 is

Pb(OH)2(s) <-------> Pb²⁺(aq) + 2OH^-(aq)

Ksp = [Pb²⁺][OH^-]² = 2.8×10^-16M³

given concentration of Pb²⁺ = 0.0496M

0.0496M × [OH^-]² = 2.8× 10^-16 M³

[OH^-]² = 5.65×10^-15M²

[OH^-] = 7.52×10^-8M

Therefore,

The concentration of hydroxide ion required to just initiate precipitation = 7.52×10^-8M