Chemistry · Electrochemistry · NEET
Faraday's first law says the mass of a substance deposited or released at an electrode is directly proportional to the amount of electric charge passed through the solution. More charge means more mass. In symbols, w ∝ Q, and since Q = It (current × time), you get w = ZIt, where Z is the electrochemical equivalent. In practice for NEET you use the ready form w = (M × Q)/(n × F), where M is molar mass, n is the number of electrons in the electrode reaction, and F = 96500 C.
1 Faraday (1 F) is the charge carried by 1 mole of electrons. Its value is 96485 C, which NEET rounds to 96500 C per mole. You get it by multiplying the charge of one electron (1.6 × 10⁻¹⁹ C) by Avogadro's number (6.022 × 10²³). So 1 F = 96500 C = charge of one mole of electrons. Remember: Faraday is a charge, not a current.
It depends on the charge (n) on the metal ion, not the metal itself. For M^n+ + n e⁻ → M, you need n Faradays for 1 mole. So Na⁺ needs 1 F, Cu²⁺ needs 2 F, Al³⁺ needs 3 F for one mole. Quick rule: Faradays = moles of metal × charge on the ion. For 20 g Ca (Ca²⁺), that is 0.5 mol × 2 = 1 F. This exact idea was asked in NEET 2020.
Use three steps. Step 1: find charge Q = I × t (current in amperes, time in seconds). Step 2: write the electrode reaction and read off n (electrons per ion). Step 3: plug into w = (M × Q)/(n × F). For example, Cu²⁺ + 2e⁻ → Cu with M = 63, n = 2, F = 96500. This single formula answers almost every NEET electrolysis numerical.
The first law links mass to charge for ONE substance: w ∝ Q. The second law compares DIFFERENT substances when the SAME charge is passed through cells in series — the masses deposited are in the ratio of their equivalent weights (E = M/n). Simple test: if the question is about one cell and asks for mass or time, use the first law. If the same current passes through two cells (like CuSO₄ and AgNO₃) and asks to compare masses, use the second law.
n is the number of electrons transferred per ion in the electrode reaction — also called the valency or the number of Faradays needed per mole. For Ag⁺ + e⁻ → Ag, n = 1. For Cu²⁺ + 2e⁻ → Cu, n = 2. For Al³⁺ + 3e⁻ → Al, n = 3. Students often forget n and get answers off by a factor of 2 or 3, which is a common NTA trap. Always balance the electrode reaction first to read n correctly.
The number of Faradays (F) required to produce 20 g of calcium from molten CaCl₂ (Atomic mass of Ca = 40 g mol⁻¹) is:
Mass in grams of copper deposited by passing 9.6487 A current through a voltmeter containing copper sulphate solution for 100 seconds is: (Molar mass of Cu = 63 g mol⁻¹, 1 F = 96487 C)
During the electrolysis of molten sodium chloride, the time required to produce 0.10 mol of chlorine gas using a current of 3 amperes is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Because it is the charge of 1 mole of electrons. Multiply the charge of one electron (1.6 × 10⁻¹⁹ C) by Avogadro's number (6.022 × 10²³) and you get about 96485 C, rounded to 96500 C for NEET calculations.
It is a charge, measured in coulombs (C), not a current (amperes). Current tells you charge per second, while 1 Faraday is a fixed total charge equal to 96500 C for every mole of electrons.
Use Faraday's second law. The same charge deposits masses in the ratio of their equivalent weights (E = M/n). This is why the same current deposits different masses of copper and silver in series cells.
Yes. Faraday's laws apply to any product at an electrode, including gases like H₂, O₂ and Cl₂. You count the electrons in the electrode reaction, for example 2Cl⁻ → Cl₂ + 2e⁻ needs 2 F per mole of Cl₂.
Forgetting the value of n (electrons per ion) or not converting grams to moles first. Always balance the electrode reaction to find n, and change mass to moles before counting Faradays.
