What Is the Approximate Concentration of Free Ni²⁺ Remaining at Equilibrium

Multiple Choice

What is the approximate concentration of free Ni²⁺ remaining at equilibrium in a solution prepared by mixing 5.0 mL of 0.10 M NiCl₂ with 5.0 mL of 2.00 M ethylenediamine? [The net formation constant for the tris(ethylenediaminenickel(II) ion is 2.1 × 10¹⁸.]

A) 0.050 M
B) 1.4 × 10^-19 M
C) 4.8 × 10^-19 M
D) 3.9 × 10^-20 M
E) 2.8 × 10^-20 M