Ease Into Prep with an OAT^® Question of the Day

The correct answer is E.

This question of cost can be translated into one of coulombs, which can then be calculated via Faraday's law. More logically, however, we can translate the question into "which of the following will require the most electrons per mole?" and look at the half reactions:

Cu²⁺ + 2 e^-→ Cu

Na⁺ + e^-→ Na

2 Cl^-→ Cl₂ + 2 e^-

2 H₂O + 2 e^-→ H₂ + 2 OH^-

Fe³⁺ + 3 e^-→ Fe

From these half reactions, it is evident that the iron(III) reduction in choice E requires that the largest number of electrons per mole of desired product be provided by the current, thus in the frame of the question, iron will be the most expensive to produce.