What Is the Approximate Concentration of Free Fe³⁺ Ion in a Solution

Multiple Choice

What is the approximate concentration of free Fe³⁺ ion in a solution prepared by mixing equal volumes of 0.06 M Fe³⁺ and 4.0 M F- solutions? [The net formation constant for FeF₅(H₂O) ₂^- is 2.0 × 10¹⁵.]

A) 2.8 × 10^-23 M
B) 6.7 × 10^-21 M
C) 6.9 × 10^-19 M
D) 4.0 × 10^-15 M
E) 8.9 × 10^-10 M