Faraday's law of electrolysis
/ FAIR-uh-day /
Imagine electroplating a spoon with silver: the longer and harder you run the current, the more silver builds up — and the relationship is beautifully exact. Pass twice the charge and you deposit twice the silver. Faraday's law of electrolysis is the precise statement of that simple proportionality: the amount of substance transformed at an electrode is directly proportional to the electric charge that passed.
Formally, Faraday's law says the moles of a species oxidized or reduced equal the total charge divided by the number of electrons per molecule times the Faraday constant — the charge carried by one mole of electrons, about 96,485 coulombs. So counting coulombs, and knowing how many electrons each molecule uses, directly counts the molecules that reacted.
It matters because this law is the rock on which coulometry and electrogravimetry stand: it lets electric charge serve as an absolute measure of amount, needing no external standard or calibration curve. The honest caveat is that it holds only if all the charge drives the intended reaction — that is, at one hundred percent current efficiency. Any side reaction, leakage, or stray process spends charge elsewhere and breaks the clean accounting.
Depositing silver needs one electron per silver ion, so to plate out one mole of silver (about 108 grams) you must pass one mole of electrons — roughly 96,485 coulombs. Pass half that charge and you get half the silver, exactly.
Amount reacted is charge divided by (electrons per molecule × Faraday constant).
The Faraday constant (about 96,485 coulombs per mole) is just the charge of one electron multiplied by Avogadro's number — it is the bridge from the electrical world (charge) to the chemical world (moles).