An aqueous solution containing Pb2+ and Cd2+ can be titrated using EDTA. If an e

Question

An aqueous solution containing Pb2+ and Cd2+ can be titrated using EDTA. If an excess of cyanide is added to the solution, the cadmium is said to be masked by the formation of a cyanide complex. Suppose 20.00 mL of a solution containing only lead and cadmium is first titrated against 45.94 mL of a 0.02000 M solution of EDTA. After adding an excess of sodium cyanide, only 34.87 mL of the same EDTA solution produces an end point. Calculate the molarities of both the lead and the cadmium in the original solution.

Explanation / Answer

Moles of EDTA cosumed by Pb^2+ = 0.02 moles/litre * 0.03487 litre = 0.0006974 moles

EDTA reacts with Pb^2+ in 1:1 ratio

so moles of Pb^2+ would be same as moles of EDTA consumed.

So moles of Pb^2+ consumed by EDTA = 0.0006974 [answer]

Total moles of EDTA consumed [before addition of cyanide] by both Pb^2+ and Cd^2+ ions = 0.020000 moles/litre *0.04594 litre

= 0.0009188 moles

EDTA reacts with both Pb^2+ and Cd^2+ in 1:1 ratio only

so moles of Cd^2+ consumed by EDTA = total moles of EDTA - moles of EDTA consumed by Pb^2+

   = 0.0009188 - 0.0006974

= 0.0002214 moles of Cd^2+ [answer]

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