Cd²⁺ was used as an internal standard in the analysis of Pb²⁺by square wave polarography. Cd²⁺ gives a reduction wave at -0.60 (±0.02) V and Pb₂₊ gives a reduction wave at -0.40 (± 0.02) V. It was first verified that the ratio of peak heights is proportional to the ratio of concentrations over the whole range employed in the experiment. Here are results for known and unknown mixtures.

The unknown mixture was prepared by mixing 25.00 (±0.05) mL of unknown (containing only Pb²⁺) plus 10.00 (±0.05) mL of 3.23 (0.01) × 10^-4 M Cd²⁺ and diluting to 50.00 (±0.05) mL.

(a) Disregarding uncertainties, find [Pb²⁺] in the undiluted unknown.

(b) Find the absolute uncertainty for the answer to part (a).

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Analyte Concentration (M) Current (pA) Known Cd?+ Pb2+ 3.23 (±0.01) x 10-5 4.18 (+0.01) X 10-5 1.64 (±0.03) 1.58 (+0.03) Unknown + Internal Standard 2+ 2.00 (+0.03) 3.00 (±0.03) ? Pb2+