What is the approximate concentration of free Ni2+(aq) remaining at equilibrium in a solution prepared by mixing 5.0 mL of 0.10 M NiCl2(aq) with 5.0 mL of 2.00 M aqeous ethylenediamine? [The net formation constant for the tris(ethylenediaminenickel(II) ion is 2.1 × 1018.]
◦ 0.050 M
◦ 1.4 × 10-19 M
◦ 4.8 × 10-19 M
◦ 3.9 × 10-20 M
◦ 2.8 × 10-20 M