The mass of copper deposited by electrolysis is proportional to which of the following?

• A. The square of current
• B. The time squared
• C. The product of current and time
• D. The inverse of current

Answer: (C)

During electrolysis, the amount of material deposited is determined by the total electric charge that passes through the cell. The charge delivered is the product of current and time, Q = I t. Faraday's law tells us that the mass deposited is proportional to this charge (m ∝ Q), with a factor that depends on the molar mass, the number of electrons transferred per atom, and Faraday’s constant. For copper, Cu2+ requires 2 electrons to become solid copper, but the key dependence remains on the total charge, making the mass deposited proportional to I t. Doubling the current or doubling the time halves or doubles the amount deposited accordingly, not to the square of either quantity or to its inverse.