A 25.00mL sample of unknown containing Fe 3+ and Cu 2+ required 16.06mL of 0.05083mol·L -1 EDTA for complete titration. A 50.00mL sample of the unknown was treated with NH 4 F to protect the Fe 3+ . Then the Cu 2+ was reduced and masked by addition of thiourea. Upon addition of 25.00mL of 0.05083mol·L -1 EDTA，the Fe 3+ was liberated from its fluoride complex and formed an EDTA complex. The excess EDTA required 19.77mL of 0.01883mol·L -1 Pb 2+ to reach an endpoint using xylenol orange. Find the concentration of Cu 2+ in the unknown.