In a steel pipe, a corrosion current in which iron dissolves as Fe → Fe²⁺ + 2e⁻ flowed at an average of 0.10 A for one year (365 days). Which value is closest to the mass of iron dissolved? Take the molar mass of Fe as 55.85 g/mol and the Faraday constant as 96485 C/mol.

Answer: B. About 0.91 kg

From Faraday's law, the dissolved mass is m = ItM/(nF). One year is 365 × 24 × 3600 = 3.15×10⁷ s, so the charge passed is Q = 0.10 × 3.15×10⁷ = 3.15×10⁶ C. Since iron goes to Fe²⁺, n = 2, and m = 3.15×10⁶ × 55.85/(2 × 96485) ≈ 913 g, about 0.91 kg. About 1.8 kg is the mistake of taking the number of electrons as n = 1. About 0.61 kg calculates with n = 3, as if iron went to Fe³⁺, which does not match the reaction given. Even a corrosion current of only 0.10 A loses nearly 1 kg of iron in a year, enough to perforate a pipe if concentrated locally.