91Ó°ÊÓ

Explain how stepwise and overall formation constants are related.

In what respect is the Fajans method superior to the Volhard method for the titration of chloride ion?

A 1.509-g sample of a \(\mathrm{Pb} / \mathrm{Cd}\) alloy was dissolved in acid and diluted to exactly \(250.0 \mathrm{~mL}\) in a volumetric flask. A \(50.00-\mathrm{mL}\) aliquot of the diluted solution was brought to a \(\mathrm{pH}\) of \(10.0\) with a \(\mathrm{NH}_{4}{ }^{+} / \mathrm{NH}_{3}\) buffer; the subsequent titration involved both cations and required \(28.89 \mathrm{~mL}\) of \(0.06950 \mathrm{M}\) EDTA. A second \(50.00-\mathrm{mL}\) aliquot was brought to a \(\mathrm{pH}\) of \(10.0\) with an \(\mathrm{HCN} / \mathrm{NaCN}\) buffer, which also served to mask the \(\mathrm{Cd}^{2+} ; 11.56 \mathrm{~mL}\) of the EDTA solution were needed to titrate the \(\mathrm{Pb}^{2+}\). Calculate the percent \(\mathrm{Pb}\) and \(\mathrm{Cd}\) in the sample.

Calamine, which is used for relief of skin irritations, is a mixture of zinc and iron oxides. A 1.056-g sample of dried calamine was dissolved in acid and diluted to \(250.0 \mathrm{~mL}\). Potassium fluoride was added to a \(10.00-\mathrm{mL}\) aliquot of the diluted solution to mask the iron; after suitable adjustment of the \(\mathrm{pH}, \mathrm{Zn}^{2+}\) consumed \(38.37 \mathrm{~mL}\) of \(0.01133 \mathrm{M}\) EDTA. A second \(50.00-\mathrm{mL}\) aliquot was suitably buffered and titrated with \(2.30\) \(\mathrm{mL}\) of \(0.002647 \mathrm{M} \mathrm{ZnY}^{2-}\) solution: $$ \mathrm{Fe}^{3+}+\mathrm{ZnY}^{2-} \rightarrow \mathrm{FeY}^{-}+\mathrm{Zn}^{2+} $$ Calculate the percentages of \(\mathrm{ZnO}\) and \(\mathrm{Fe}_{2} \mathrm{O}_{3}\) in the sample.