What Is the Approximate Concentration of Free Fe²⁺(aq)remaining at Equilibrium

Multiple Choice

What is the approximate concentration of free Fe²⁺(aq) remaining at equilibrium in a solution prepared by mixing 5.0 mL of 0.10 M FeSO₄(aq) with 5.0 mL of 2.0 M KCN(aq) ? [The net formation constant for the hexacyanoferrate(II) ion is 1 × 10³⁷.]

A) 3 × 10^-22 M
B) 7 × 10^-30 M
C) 4 × 10^-38 M
D) 5 × 10^-37 M
E) 4 × 10^-40 M