A student investigates an aqueous solution of an unknown divalent metal ion M by…
CHEMISTRY · Multi-part · AP · Chemistry · Thermodynamics and Electrochemistry · Thermodynamics and Electrochemistry
Problem
A student investigates an aqueous solution of an unknown divalent metal ion M2+ by electrodepositing the metal onto a pre-weighed platinum cathode at constant current. After the electrolysis the cathode is rinsed, dried, and weighed to determine the mass of metal deposited.
Assume the Faraday constant is 96 485 C mol−1 of electrons.
(a) Describe the quantitative relationship that must exist between the electrical charge passed through the cell and the number of moles of M deposited on the cathode.
(b) Explain why the oxidation state of the metal ion in solution determines the ratio between the number of moles of electrons transferred and the number of moles of metal deposited.
(c) Outline the sequence of calculations that would allow the student to determine the molar mass of the unknown metal from the measured current, the time elapsed, and the change in mass of the cathode.
(d) Identify two systematic experimental errors that could lead to an overestimation of the molar mass of the metal, and for each error state whether it would make the calculated molar mass larger or smaller than the true value, justifying your answer.