Question

A 25.00 mL sample of unknown containing Fe₃⁺ and Cu₂⁺ required 16.06 mL of 0.05083 M EDTA for complete titration. A 50.00 mL sample of the unknown was treated with NH₄F to protect the Fe₃⁺. Then the Cu₂⁺ was reduced and masked by the addition of thiourea. On addition of 25.00 mL of 0.05083 M EDTA, the Fe₃⁺ was liberated from its fluoride complex and formed an EDTA complex. The excess EDTA required 19.77 mL of 0.01883 M Pb₂⁺ to reach a xyenol orange end point. Find [Cu₂⁺] in the unknown.

[Cu₂⁺] =

Answer 1

To calculate the concentration of Cu²⁺, subtract the moles of EDTA that reacted with Fe³⁺ from the total moles of EDTA and divide by the volume of the original sample.

Step-by-step explanation:

To find the concentration of Cu²⁺ in the unknown, we need to consider the titrations performed and their outcomes. The 25.00 mL sample initially titrated with 0.05083 M EDTA reacts with both Fe³⁺ and Cu²⁺. The molarity of EDTA and the volume used gives us the total moles of EDTA that reacted with both metal ions:

Total moles of EDTA = Molarity of EDTA * Volume of EDTA = 0.05083 mol/L * 0.01606 L = 0.0008167 moles

In the second step, after treating the 50.00 mL sample with NH₄F and thiourea, only Fe³⁺ responds to the additional 25.00 mL of 0.05083 M EDTA since Cu²⁺ is masked. Any excess EDTA was back titrated with 0.01883 M Pb²⁺. To find this excess, we calculate the moles of Pb²⁺ used:

Moles of Pb²⁺ = Molarity of Pb²⁺ * Volume of Pb²⁺ = 0.01883 mol/L * 0.01977 L = 0.00037257 moles

Since one mole of Pb²⁺ reacts with one mole of EDTA, the moles of EDTA that reacted with Fe³⁺ only can be found by subtracting the moles of Pb²⁺ from the total moles of EDTA:

Moles of EDTA that reacted with Fe³⁺ = Total moles of EDTA - Moles of Pb²⁺ = 0.0008167 - 0.00037257 = 0.00044413 moles

Therefore, the moles of EDTA that originally reacted with Cu²⁺ in the first titration are:

Moles of EDTA with Cu²⁺ = Total moles of EDTA - Moles of EDTA with Fe³⁺

Finally, divide the moles of EDTA that reacted with Cu²⁺ by the volume of the original sample (25.00 mL or 0.025 L) to find the concentration of Cu²⁺ in the unknown:

[Cu²⁺] = Moles of EDTA with Cu²⁺ / 0.025 L