Chemistry · Electrochemistry · NEET
Always use the same 4-step chain. Step 1: find charge Q = I × t (current in amperes, time in seconds). Step 2: moles of electrons = Q / 96500. Step 3: use the electrode reaction to find moles of product (divide electrons by n, the electrons needed per atom). Step 4: mass = moles of product × molar mass. Never skip converting time to seconds first.
96500 coulombs is the charge carried by 1 mole of electrons. This number is the Faraday constant (NCERT uses 96487, but NEET usually rounds to 96500 or 96000). So dividing the total charge Q by 96500 tells you how many moles of electrons flowed. It is just a unit conversion: coulombs to moles of electrons.
Look at the ion's charge in the electrode half-reaction. Cu²⁺ + 2e⁻ → Cu needs n = 2. Ag⁺ + e⁻ → Ag needs n = 1. Al³⁺ + 3e⁻ → Al needs n = 3. For chlorine at the anode, 2Cl⁻ → Cl₂ + 2e⁻, so 1 mole of Cl₂ gas needs 2 moles of electrons. n is simply the number of electrons in the balanced half-reaction.
They are not the same. Moles of electrons come straight from Q / 96500. Moles of product = moles of electrons ÷ n. Example: if 2 moles of electrons pass and you deposit Cu (n = 2), you get only 1 mole of Cu. Students lose marks by treating electrons and metal atoms as equal. Always divide by n.
Reverse the chain. Start from moles of product, multiply by n to get moles of electrons, multiply by 96500 to get charge Q, then use t = Q / I. This is exactly the 2016 NEET chlorine question: 0.10 mol Cl₂ needs 0.10 × 2 × 96500 = 19300 C, and t = 19300 / 3 ≈ 6433 s ≈ 110 minutes.
First find total charge Q = I × t. Then divide by the charge of one electron (1.6 × 10⁻¹⁹ C), not by 96500. Number of electrons = Q / (1.6 × 10⁻¹⁹). Use 96500 only when you want moles of electrons. Read the question: 'number of electrons' means use 1.6 × 10⁻¹⁹.
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
The number of electrons delivered at the cathode during electrolysis by a current of 1 ampere in 60 seconds is (charge on electron = 1.60 × 10⁻¹⁹ C)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
One Faraday is the charge of 1 mole of electrons. Its value is 96487 C/mol in NCERT, usually rounded to 96500 C/mol in NEET numericals.
Q = I × t, where Q is charge in coulombs, I is current in amperes, and t is time in seconds. Always convert minutes or hours to seconds first.
By Faraday's first law, mass deposited is directly proportional to the charge passed. In steps: mass = (Q / 96500) ÷ n × molar mass, where n is electrons per atom.
Use 96500 when the answer should be in moles of electrons. Use 1.6 × 10⁻¹⁹ C when the question asks for the actual number of individual electrons.
Electrolysis numericals appear almost every year and are quick, guaranteed marks if you follow the Q = It chain. They test unit conversion and the electrode half-reaction, both easy to master.
