First law: The amount of the substance liberated or deposited or dissolved at an electrode during electrolysis of an electrolyte is directly proportional to the quantity of electricity passing through the solution of electrolyte or melt. Mathematically Faraday’s first law is \(m \propto q\) or \(m = eq\) or \(m = ect\), where m is the mass of the substance liberated or deposited or dissolved, q is the quantity of electricity in coulomb, t is the time in seconds, c is the strength of current in amperes and e is the electrochemical equivalent of the ion or metal or molecule deposited or liberated or dissolved at the electrode.
Chemical equivalent of a substance is the amount of substance deposited or liberated or dissolved or had undergone electrode reaction at an electrode during the passage of one Faraday of electricity during the electrolysis of electrolyte solution or melt.
\(\text{Chemical equivalent of an element or ion = }\frac{\text{Atomic weight}}{\text{Valency or charge of the ion}}\)
Second law of Faraday states that if same quantity of electricity is passed through different electrolyte solutions or melts, the amount of the different substances liberated or deposited or dissolved or had undergone reaction at electrode is directly proportional to their chemical equivalents.
\( \displaystyle \frac{W_1}{E_1} = \frac{W_2}{E_2} = \frac{W_3}{E_3} \)
The quantity of electricity that can deposit 108 g of silver from AgNO3 solution is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 1 Faraday
One Faraday (1 F ≈ 96500 C) is the charge on one mole of electrons. In the passage, the chemical equivalent of a substance is the amount deposited by one Faraday, equal to the atomic weight divided by the charge of the ion.
At the cathode each silver ion takes one electron:
\( \displaystyle \ce{Ag^+(aq) + e^- -> Ag(s)} \)
108 g of silver is 1 mol of Ag (molar mass 108 g mol−1), so it needs 1 mol of electrons, that is 1 F. The chemical equivalent of silver is 108/1 = 108 g, which is the same result.
Why the other options are incorrect
Option (1) 1 ampere — Incorrect. The ampere is the unit of current, not of quantity of electricity; the charge passed is current × time (Q = It).
Option (2) 1 coulomb — Incorrect. 1 C is far too small: it deposits only the electrochemical equivalent of silver, 108/96500 ≈ 1.12 × 10−3 g.
NCERT (2026-27 edition):
2.5 Electrolytic Cells and Electrolysis – “The amount of electricity (or charge) required for oxidation or reduction depends on the stoichiometry of the electrode reaction. For example, in the reaction:
\( \displaystyle \ce{Ag^+(aq) + e^- -> Ag(s)}\qquad(2.30) \)
One mole of the electron is required for the reduction of one mole of silver ions.
We know that charge on one electron is equal to 1.6021 × 10–19 C. Therefore, the charge on one mole of electrons is equal to:
\( \displaystyle N_A\times1.6021\times10^{-19}\,\mathrm{C}=6.02\times10^{23}\,\mathrm{mol^{-1}}\times1.6021\times10^{-19}\,\mathrm{C}=96487\,\mathrm{C\,mol^{-1}} \)
This quantity of electricity is called Faraday and is represented by the symbol F.”