A 25.00-mL sample containing Fe3[1] and Cu²⁺ required 16.06 mL of 0.050 83 M EDTA for complete titration. A 50.00-mL sample of the unknown was treated with NH4F to protect the Fe³⁺. Then Cu²⁺ was reduced and masked by thiourea. Addition of 25.00 mL of 0.050 83 M EDTA liberated Fe³⁺ from its fluoride complex to form an EDTA complex. The excess EDTA required 19.77 mL of 0.018 83 M Pb²⁺ to reach a xylenol orange end point. Find [Cu²⁺] in the unknown.