Determine the concentration of Ni²⁺ that remains when a solution of 1.3 × 10^-3 M Ni(NO₃)₂ and 0.173 M NH₃ reaches equilibrium.
K_f (Ni(NH₃)₆²⁺) = 1.2 × 10⁹

• A. 4.0x10^-8
• B. 6.4x10^-12
• C. 5.3x10^-8
• D. 1.1x10^-12

Answer: (C)