In excess of NH3(aq), Zn2+ forms a complex ion, [Zn(NH3)4]2+ which has a formation constant Kf = 7.8 x 10^8. Calculate the concentration of Zn2+ in a solution prepared by adding 1.00 x 10^-2 mol Zn(NO3)2 to 1.00 L of 0.250 M NH3. (a) 7.9 x 10^-4 M (b) 2.8 x 10^-6 M (c) 3.9 x 10^-9 M (d) 6.4 x 10^-11 M